Clean the mushrooms thoroughly with a damp cloth or paper towel; avoid rinsing them under water to prevent sogginess. Slice the mushrooms into even pieces, about 1/4 inch thick, and set them aside.

Mince the garlic cloves finely and chop the fresh parsley, preparing all the aromatics for later steps.

Heat a large skillet or frying pan over medium-high heat, then add 2 tablespoons of unsalted butter and 1 tablespoon of olive oil. Once the butter is melted and shimmering, add the sliced mushrooms to the hot pan in a single layer, ensuring not to overcrowd the skillet.

Sauté the mushrooms for 5-7 minutes without stirring initially, allowing them to brown nicely, then stir occasionally until they release their moisture and turn golden brown.

Push the sautéed mushrooms to one side of the pan, add the remaining 2 tablespoons of butter and the minced garlic to the empty side, and cook for about 1 minute until the garlic is fragrant.

Stir the garlic and butter thoroughly into the mushrooms, ensuring everything is well combined and coated.

Season the mushrooms with salt and black pepper to taste, then remove the skillet from the heat and stir in the fresh chopped parsley.

Serve the garlic butter sautéed mushrooms immediately as a delightful side dish or an elegant appetizer.