Preheat oven to 350 degrees F (175 degrees C). Coat a 9 inch square baking dish with cooking spray.
Bring a large pot of salted water to a boil. Add potatoes and cook until tender enough to pierce with a fork; drain. Mash by hand using a potato masher or sturdy whisk. Mix in butter, milk, garlic powder, and salt and pepper to taste. Set aside.
Heat oil in a large skillet over medium heat. Add onions; cook and stir until softened, 8 to 10 minutes. Add garlic and ground beef and cook until meat loses its pink color. Season with seasoned salt and taco seasoning. Add peas and corn and stir until blended.
Pour beef and vegetable mixture into prepared baking dish. Cover with mashed potatoes.
Bake in preheated oven until potatoes are golden and beef and vegetable mixture is hot and bubbly, 30 to 35 minutes.
