Preheat oven to 400°F (200°C).
In a large skillet, heat olive oil over medium heat. Add onions and garlic, sauté until soft.
Add ground beef. Cook until browned, breaking it apart with your spoon or spatula.
Stir in tomato paste, salt, pepper, and flour. Cook for 2 more minutes.
Pour in beef broth and bring to a simmer. Add frozen peas and carrots. Simmer for 5 minutes until slightly thickened.
Spread the beef mixture in an 8x8-inch baking dish.
Top evenly with mashed potatoes. If using cheese, sprinkle it over the top.
Bake for 20 minutes or until the top is golden and the edges are bubbling.
Let cool slightly before serving—trust me, your tongue will thank you.