Combine olive oil, onion, bell peppers, tomatoes, broth, salt and garlic in a crockpot. Cook on low for 6-8 hours or high for 3-4 hours.
Preheat oven to 450F.
Line a baking sheet with parchment paper and place bread slices on it.
Spread butter on 3 slices of bread. Place cheese on the other 3 slices.
Bake for 5 minutes, remove from oven, and combine into sandwiches.
Bake for another 4-5 minutes.
Remove from oven, cut into small squares, and serve on top of soup.