Preheat oven to 350 degrees. Scoop out the insides of the tomatoes, leaving just the shell, and discard. Turn upside down on a paper-towel lined plate for about 5 minutes to let any excess fluid drip out.
Saute the onions and mushrooms on medium high heat in a small skillet with about 4 tablespoons of butter. Once the onions are soft and translucent, remove from heat and let cool.
Turn the tomatoes cut side up and place on a lightly greased cookie sheet lined with aluminum foil. Season the tomatoes with a little garlic salt and Italian seasoning.
Combine the ricotta, mozzarella, bacon, onions and mushrooms in a small bowl. Stuff the tomatoes with the cheese mixture and gently sprinkle more garlic salt and Italian seasoning over the tomatoes.
Bake for approximately 20 minutes. Drizzle with balsamic vinaigrette dressing and freshly grated Asiago cheese, if desired.