Preheat oven to 400°F. Wrap garlic cloves in a tight foil packet. Place tomatoes, cut side up, on large baking sheet. Sprinkle generously with salt and pepper. Drizzle tomatoes with olive oil. Add foil packet of garlic to tray. Roast until tomatoes are brown and tender, about 1 hour. Cool slightly.
Unwrap garlic packet and peel cloves. Transfer cloves, tomatoes and any accumulated juices to a blender or food processor and pulse machine on and off until tomatoes are a chunky puree. Transfer tomatoes to medium pot and add thyme, crushed red pepper and stock and bring to a boil. Reduce heat to a simmer and cook, uncovered, for 25 minutes. Remove from heat and adjust seasonings to taste.
Preheat oven to 350. Arrange four ovenproof soup bowls, crocks or large mugs on a large, foil-lined baking sheet. Stir grated onion into the warm soup. Float toast slice(s) in each bowl, buttered side up and divide grated cheese generously over top. Bake soups on tray for 15 to 20 minutes, until cheese on top is bubbling and brown at the edges. If you’d like it even more bronzed on top, preheat your broiler and finish soups for a minute or two under it. Serve immediately.
Soup can be prepared one day ahead, and kept covered in the fridge. Rewarm before serving, or before finishing with cheddar crouton.
