Combine first 4 ingredients in a food processor; process until finely chopped. Heat a large saucepan over medium-high heat. Add oil to pan; swirl to coat. Add vegetable mixture to pan; cook 5 minutes or until crisp-tender, stirring occasionally. Add tomatoes to food processor; pulse until finely chopped. Add tomatoes, stock, 1/2 teaspoon pepper, and salt to pan; bring to a simmer. Reduce heat to low; simmer 10 minutes. Stir in butter; sprinkle with celery leaves.
Place bread on a baking sheet. Broil 2 minutes. Turn bread slices over; sprinkle evenly with cheese. Broil 2 minutes or until cheese is lightly browned. Sprinkle with remaining 1/4 teaspoon black pepper.