Sprinkle some salt onto a plate and roll the potatoes in the salt.
Place potatoes directly on the oven center rack. You might want to put a baking sheet lined with foil beneath them to catch any drippings.
Bake until potatoes are done (210 F internal temperature, or tender all the way through when poked with a knife), 40-60 minutes depending on their size. You can also test for doneness by squeezing the potatoes (with potholders on). If they give easily, they are done.
When ready to serve, poke the tops of the potatoes with a fork in cross pattern (i.e. from edge-to-edge and top-to-bottom). Pretend you are tracing the lines you would make if cutting the potatoes with a knife. Then, grab the potatoes from the ends and squeeze. The potatoes will open up perfectly. Fluff with the fork before adding the mushroom mixture.
Top with cheese if desired. Place the potatoes under the broiler for a minute or two if you want to melt and toast the cheese.
For the mushroom mixture
Melt butter in a large skillet over medium-high heat.
Stir in the Worcestershire sauce.
Add the mushrooms and cook until they start to soften, about 5 minutes, stirring gently on occasion.
Add the onions and stir. Cook another 5 minutes, stirring gently on occasion, until the onions are just softened.
Reduce heat to low. Add the garlic and season with salt and pepper, to taste. Stir and cook another 2-3 minutes or until the garlic is cooked.