How to Make It
Brown beef in a large nonstick skillet over medium-high heat, stirring often, 10 minutes or until meat crumbles and is no longer pink. Remove beef from skillet using a slotted spoon; reserve 2 tablespoons drippings in skillet. Reduce heat to medium.
Sauté onion, mushrooms, and garlic in hot drippings over medium heat 10 to 11 minutes or until tender. Stir in ground beef, peas, and next 4 ingredients. Sprinkle flour over meat mixture. Increase heat to medium-high, and cook, stirring constantly, 1 minute. Stir in tomatoes, bay leaf, and vinegar, breaking up large tomato pieces with a spoon. Reduce heat to medium, and cook, stirring often, 3 minutes or until slightly thickened. Remove bay leaf. Transfer mixture to a lightly greased 3-quart ovenproof skillet or baking dish.
Prepare mashed potatoes according to package directions; let cool. Spoon cooled potatoes over beef mixture in skillet, smoothing gently with back of spoon.
Bake at 400°F for 15 minutes or until thoroughly heated. Let stand 5 minutes before serving.
For testing purposes only, we used Simply Potatoes frozen mashed potatoes from Crystal Farms.
To make ahead: Prepare recipe as directed through Step 3. Cover tightly with aluminum foil. Freeze up to 1 month. Thaw in refrigerator 24 hours. Bake at 400°F for 40 minutes or until thoroughly heated, shielding with aluminum foil after 25 minutes to prevent excessive browning.