To prepare soup, melt butter in a large saucepan over medium heat. Add onion; sauté 3 minutes. Add tomato; cook, covered, over medium heat 10 minutes. Uncover; cook until mixture is reduced to 2 cups (about 25 minutes), stirring occasionally.
Place the tomato mixture in a blender, and process until smooth. Combine the tomato mixture, milk, salt, and pepper in pan, and cook mixture over medium-low heat 2 minutes or until heated.
To prepare croutons, rub cut sides of garlic over both sides of each bread slice; place bread on a baking sheet. Broil 30 seconds on each side or until toasted. Cut each slice in half crosswise. Place 1 cup soup in each of 4 ovenproof bowls; top each serving with 1 crouton and 2 tablespoons cheese. Broil 2 minutes or until cheese is melted.