Preheat oven to 375 degrees F. Coat a 3-quart rectangular baking dish with cooking spray; set aside.
In a large skillet heat 3 tablespoons of the butter over medium heat until melted. Add carrots, celery, onions, and pepper. Cook for 7 to 9 minutes or just until vegetables are tender, stirring occasionally. Remove skillet from heat. Add mushrooms, Burgundy, mustard, garlic, rosemary, and thyme. Return skillet to heat. Bring to boiling; reduce heat. Simmer, uncovered, for 5 minutes. Stir in beef with gravy and broth; heat through. Cover and keep warm.
In a small microwave-safe bowl microwave the remaining 1 tablespoon butter on 70 percent power (medium-high) about 30 seconds or until melted. Brush cut sides of croissants with melted butter; cube croissants. Transfer hot meat mixture to the prepared baking dish. Sprinkle with cubed croissants. Bake, uncovered, for 20 to 25 minutes or until filling is bubbly and croissants are golden.