Coat the onion steaks and tomatoes with olive oil, cut the top off the garlic and drizzle with olive oil. Roast, turning/flipping occasionally on a hot BBQ until lightly charred. Place the garlic in the coolest area of the grill and don't worry about it until you remove everything from the grill.
Let the tomatoes cool a bit then remove the green stem core and skins.
Rough chop the tomatoes and onions and place in a large sauce pot.
Add the rest of the ingredients (except salt!)
Bring to a simmer and bring out the hand blender (or food processor or blender)
Bring back to a simmer and taste for any needed salt and add if needed.