Wipe mushrooms clean with a damp paper towel and cut each mushroom in half lengthwise.

Use fresh paper towels to pat mushroom halves completely dry on both sides to ensure proper browning.

Place a large skillet over medium-high heat for about 2 minutes until hot, then add 1 tablespoon of butter and 1 tablespoon of olive oil, swirling to coat the pan.

Arrange mushroom halves in a single layer, flat side down, leaving space between each piece and cook undisturbed for 4 to 5 minutes until deep golden-brown.

Flip the mushrooms, add the remaining 2 tablespoons of butter, and cook for another 2 to 3 minutes until the second side is browned and butter is foaming.

Reduce heat to medium and add minced garlic, stirring constantly for about 30 seconds until fragrant; then sprinkle in kosher salt, black pepper, and fresh thyme and stir briefly.

Remove the pan from heat and stir in chopped parsley and balsamic vinegar if using; taste and adjust seasoning with additional salt or pepper.