Preheat your oven to 350°F (175°C).
In a skillet, brown the ground beef and onions over medium heat until the meat is no longer pink.
Once cooked, drain any excess fat and season with garlic powder, salt, and pepper.
In a large bowl, whisk together the milk and eggs.
Stir in the cooked beef mixture, cheese, flour, diced tomatoes, and baking powder into the milk and egg mixture.
Pour the filling into a greased pie dish and smooth the top.
Bake in the preheated oven for 30–40 minutes until the top is golden brown and slightly puffed.
Let the pie cool for a few minutes before slicing and serving.