In a large skillet, heat the butter over medium heat.
When the foaming subsides, add the sliced mushrooms, minced garlic, kosher salt, black pepper, and dried thyme.
Cook, stirring often, over medium heat, until the mushrooms are tender and the liquids they release have evaporated, for about 7 minutes.
Garnish the mushrooms with chopped parsley. Serve immediately.
Video
Notes
The most important tip I can give you is to be generous with the butter and stay patient. Mushrooms are best when cooked in lots of fat, over medium heat, slowly and patiently, until very tender. That's when the mushrooms release their flavor.
It's best to wipe the mushrooms clean with damp paper towels. If you prefer to wash them, do so quickly (don't soak them) and dry them well right after washing.
The leftovers keep well in the fridge, in an airtight container, for 3-4 days. In fact, they tend to become even more flavorful after a day or two! You can reheat them gently, covered, in the microwave at 50% power.