Melt butter over medium high heat in a medium skillet then add the sliced onions. Cook for 5 minutes until softened.
Clean the mushrooms with a brush, then slice into thick slices (or if they're smaller, just cut in half).
Add to the skillet with the onions and cook for 5 minutes, without stirring.
Add the garlic and cook another 2 minutes.
Add the brown sugar, Worcestershire sauce and black pepper and cook a further 5 minutes without stirring.
Remove from the heat and sprinkle with chopped parsley, for garnish.
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.