Use a damp paper towel to brush any dirt off of the mushrooms. (Note 1)
Cut into thin slices. Heat a large skillet over medium heat. Add 1 tablespoon of butter. When melted add the sliced mushrooms.
Cook, stirring occasionally, for 8-10 minutes until brown. Season with salt and pepper and thyme. Stir together and transfer to a plate.
Onions
While the mushrooms are cooking, slice the onions into thin slices.
Add the remaining 2 tablespoons of butter to the same skillet used to cook the mushrooms. Add the onions, stirring to coat with the butter. Reduce the temperature to medium-low and cook for 20-30 minutes until soft and golden brown. (Note 2)
Season with salt and pepper. Add the mushrooms back to the pan and toss with the onions. (Note 3)
Notes
Do not clean mushrooms by dipping in water or holding them under running water. They are very porous and will become water-logged and soggy.
If the onions start to dry out, add 1-2 tablespoons of water as needed.
Stir in 1 teaspoon of Worcestershire sauce for added flavor if desired.
Store in an airtight container in the refrigerator for 3-4 days.