For each tomato: Halve, core and deseed halves. Sprinkle cut sides with salt. Lay halves on paper towels to drain, cut sides down.
Wait 20 minutes. Preheat oven to 350 F. Lightly coat oven dish with olive oil. Chop onion, garlic, and mushrooms to desired chunkiness, small enough to stuff into tomato halves.
For each tomato half, with cut side up: Stuff with onion, garlic, and mushrooms. Sprinkle with salt and spices. Drizzle with olive oil, brushing to distribute evenly. Place in dish.
Bake for 1:15. Serving suggestion: on bread or over pasta.
Jon Stewart