Brown the ground beef in a large skillet over medium-high heat, about 6-8 minutes. Drain excess grease.
Add onion, carrot, and celery; cook 5 minutes. Add garlic; cook 30 seconds more.
Sprinkle flour over the mixture, stir, cook 1 minute. Stir in tomato paste, beef broth, and Worcestershire sauce. Simmer 5 minutes until thickened.
Stir in peas. Season with salt and pepper.
Preheat oven to 400°F. Transfer filling to a baking dish (or use an oven-safe skillet). Top with mashed potatoes, spreading to the edges.
Sprinkle with cheddar, if using.
Bake 20-25 minutes until bubbling and lightly golden. Broil 2-3 minutes for extra color, if desired.
Rest 5-10 minutes before serving.