Heat butter in a medium, heavy-bottomed saucepan over medium-high heat. Add onions and cook for several minutes, stirring occasionally, until the edges begin to turn slightly brown.
Add mushrooms, cover with a lid and cook for two to four minutes, turning mushrooms over once or twice, until the mushrooms are turning golden brown and have given up a little of their juices.
Remove lid, add garlic and stir. Sprinkle with 1/2 teaspoon (or to taste) of salt and a few grinds of pepper and cook for a minute or two longer, until the juices have started to evaporate and concentrate. Add Worcestershire sauce, stir and remove from heat.
If for some reason, your mushrooms are very juicy, they can be removed from the pan, leaving the juices behind. Crank the heat and reduce the juices for a minute until there are just a couple tablespoons left, turn the heat down, add the mushrooms back in and proceed with the recipe.