Preheat oven to 350°. In a large skillet, cook and crumble beef, salt and pepper until beef is no longer pink. Drain beef; remove from skillet.
In same skillet, heat oil over medium heat. Add onion; cook and stir until tender, 2-3 minutes. Add garlic; cook 1 minute longer.
Add beef, mixed vegetables, kidney beans, tomatoes, beef stock and chili seasoning; bring to a boil. Cook and stir until thickened, about 5 minutes. Transfer to a greased 13x9-in. baking dish. Heat mashed potatoes according to microwave package directions; spread over top of beef mixture.
Sprinkle with cheese; bake until bubbly and cheese is melted, 15-20 minutes. Cool for 5 minutes; sprinkle with green onions and Parmesan cheese.
Test Kitchen tips
Cottage pie or shepherd's pie? Traditionally, shepherd's pie is made with lamb, while cottage pie is the same dish but with beef.
Black beans taste great in this dish if you're not a fan of kidney beans.
For a lighter take, substitute ground chicken or turkey for the beef.
Add chopped jalapenos or a splash of your favorite hot sauce for a spicy touch.