Preheat oven to 375°. Lightly coat 8-9 in. baking dish with olive oil.
Hollow out the tomatoes by cutting of their tops and scooping out the insides with a spoon. Dice the insides and transfer to a mixing bowl.
Add a pinch of salt (1/8 tsp) to the inside of each hollow tomato. Set upside-down on a plate to drain.
Fry up the onions until soft. Add garlic during the last 30 seconds. Add diced tomato and 1/2 tsp salt, and cook, stirring occasionally, until almost all of the liquid has evaporated, ~5 min.
Remove from heat and add remaining ingredients. Season to taste with pepper. Mix well.
Stuff tomatoes with filling. Add the tomato top as a little lid!
Bake 30 min.