Heat olive oil in a Dutch oven over medium-high heat. Add chopped onion, mushrooms, carrot, and bell pepper; cover and cook for 3 minutes. Stir in herbes de Provence, 1/4 teaspoon black pepper, broth, and tomatoes. Bring to a boil; cover, reduce heat, and simmer 5 minutes. Stir in navy beans; cover and simmer 5 minutes.
While soup cooks, arrange the bread slices on a baking sheet. Sprinkle evenly with cheese. Broil 2 minutes or until cheese melts. Spoon 1 3/4 cups soup into each of 4 bowls, and top each serving with 2 bread slices.