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In a skillet, over medium heat, add the butter.
2 tablespoons butter or vegetable oil
Once the skillet is hot, add the onion and cook until soft and starting to brown, 4-5 minutes.
¾ cup finely chopped yellow onion
Stir in the garlic and cook for one more minute.
3-4 minced garlic cloves
Add the sliced mushrooms and cook until soft, about 5 minutes, stirring occasionally.
300 grams sliced mushrooms
Mix in the thyme, salt, and black pepper.
2 teaspoons thyme, dry or fresh, 1 teaspoon salt, ¼-½ teaspoon black pepper
Pour in the chicken stock and lower the heat to between medium and low. Cover with a lid. Let it simmer for 4-5 minutes.
2 cups chicken stock
In a bowl, add the cornstarch or flour, add a few tablespoons of the cooking liquid, and whisk until there are no lumps. Pour it back into the mushroom sauce, stir, and cook for 1-2 more minutes.
2 tablespoons cornstarch, flour, or potato starch
Lastly, add the parmesan cheese, stir, and remove from the heat. Use as needed.
⅓ cup grated parmesan cheese