Caramelized Onions and Mushrooms Recipe

There’s something quietly luxurious about a skillet of onions and mushrooms slowly caramelizing on the stove. In my kitchen, this simple side dish often steals the show. It’s not flashy, but it lingers in your memory, especially when nestled beside a perfectly cooked steak or layered onto a juicy burger. If you’ve ever wondered how to bring restaurant-level depth to your home cooking, this is one of those recipes that delivers—rich, savory, and deeply satisfying.

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Growing up, caramelized onions and mushrooms weren’t on the menu every time we had steak because my mom thought the effort to make them wasn’t worth it; I disagree. This recipe is simple and relatively hands off. The onions and mushrooms need heat and time to caramelize. You can prepare the rest of your meal while the Maillard reaction does its thing.

What is the Maillard Reaction?

The Maillard reaction (pronounced “my-YARD”) is the fancy name for what happens when food browns and develops rich, deep flavor as it cooks. It’s what gives seared steak that golden crust and turns onions and mushrooms sweet and savory as they caramelize.

It happens when proteins and natural sugars in food react to heat, usually on a surface like a hot pan. It’s not burning, and it’s not just caramelizing sugar; it’s a special browning that adds flavor, color, and that irresistible “something” we all love in well-cooked food.

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Ingredient List

Mushrooms: I use white button mushrooms, but you can substitute baby bellas, oyster, and shitake mushrooms as well. Don’t be afraid to wash them in water and allow the water to release and cook off.  

Fermented Garlic: If you haven’t already, you really should make my fermented garlic. It is easy and you’ll always have peeled garlic on hand. If you don’t have fermented garlic, you can use raw garlic. 

Flavor Variations

This is my foundational recipe, but if you want to deepen the flavor, try adding a tablespoon of fish sauce or soy sauce. Both are rich in umami and will take the mushrooms from simple to stunning.

The best moment to introduce them is after the mushrooms have released their moisture and the pan begins to look dry. At that point, the mushrooms are soft and ready to soak in every drop of flavor, becoming little umami bombs in the process.

Don’t let the scent of fish sauce scare you off. While it smells assertive, it doesn’t taste fishy once added. Instead, it melts into the dish, leaving behind a savory depth that makes everything taste more alive.

Step by Step Instructions

1. Heat a cast iron or stainless steel skillet for 10 minutes on medium heat
2. Add butter, avocado oil, and onions and stir
3. Cook for 5 minutes to allow the onions a head start in caramelizing and softening
4. Add mushrooms, salt, pepper, garlic and onion powder, and cayenne and stir
5. Allow the mushrooms to cook for 10 minutes. During this time, the water in the mushrooms will release. Allow it to cook off before the next step.

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6. Add garlic and stir. Continue to cook on medium heat for 5 more minutes, stirring occasionally to prevent burning.
7. Turn the temperature to medium low (between 2 and 3 on my stove) and allow to slowly caramelize until softened and brown around the edges of the mushrooms. This process takes around 20 minutes
8. Stir and check to see if the mushrooms need to cook any further. They should be tender, brown, and slightly crisped around the edges
   1. If they look like the mushrooms in my photos, they’re done
   2. If not, cook without disturbing in five minute increments until they look good

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Tips and Tricks

Don’t worry about the water absorption that happens when you wash your mushrooms. I’ve seen recommendations to only wipe the dirt off to avoid water, but I don’t do that. I prefer to wash the dirt off. The water from the mushrooms will seep out. You do want to cook the water off to ensure browning. Simply, let the mushrooms cook at medium high until the water has released, then evaporated. At this point adding soy or fish sauce will allow it to absorb into the mushrooms.

Try to let the onions and mushrooms cook without disturbing them. The Maillard reaction works best when the heat is continual. 

Serving Suggestions

My favorite way to serve caramelized onions and mushrooms is with dry brined steak. They’re the perfect accompaniment. 

Piled high on a mushroom and Swiss cheese burger is another decadent way to use these mushrooms and onions. 

Another way I use them is when cooking chicken thighs. I cook them almost all of the way, then add the chicken before cooking it in the oven. It gives almost a chicken marsala flavor.

Storage and Reheating

I store any leftover mushrooms and onions in the refrigerator in a container for up to 3 days. I will either microwave them or if I heat leftover steak on the skillet, I’ll warm them up with it.

FAQs

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There’s something magical about the smell of mushrooms and onions slowly caramelizing while the steak rests nearby—it’s the kind of side dish that invites you to linger a little longer at the table. If you try this recipe, please leave a comment to let me know how it turned out.