Melt butter over medium high heat in a medium skillet then add the sliced onions. Cook for 5 minutes until softened.
2 Tablespoons Salted Butter, ½ Each Yellow Onions
Clean the mushrooms with a brush, then slice into thick slices (or if they're smaller, just cut in half).
8 Ounces White Button Mushrooms
Add to the skillet with the onions and cook for 5 minutes, without stirring.
Add the garlic and cook another 2 minutes.
2 Cloves Garlic
Add the brown sugar, Worcestershire sauce and black pepper and cook a further 5 minutes without stirring.
1 teaspoon Light Brown Sugar, 1 Tablespoons Worcestershire Sauce, ½ teaspoon Black Pepper
Remove from the heat and sprinkle with chopped parsley, for garnish.
½ teaspoon Fresh Parsley
Notes
It's important not to move the mushrooms around too much when cooking or they'll become soggy. If you're able, pick up the skillet and 'pan-flip' the mushrooms once or twice.