Combine tomatoes, onion, garlic and olive oil on a baking sheet and toss to coat. Bake until tomatoes burst and you start to see blisters form, and onions are slightly brown; about 25 minutes.
Cut avocados in half and remove pits. Optional step: Preheat grill to medium heat. Grill on lightly oiled grill, until warmed through and grill marks are visible; about 10 minutes.
Allow all ingredients to cool enough to handle.
Remove avocado flesh from peel, and mash with lime juice.
Mince roasted onions and garlic and add to avocado mash, along with tomatoes. Mash all together.
Add chopped cilantro and salt and pepper to taste. Stir. Enjoy warm or cold!