Heat oil over medium-high heat in a large cast-iron or broiler safe nonstick skillet. Add beef and sprinkle with half each of the salt and pepper; cook, stirring often and using back of spoon to break up meat into small chunks, for 8 to 10 minutes or until meat is browned and thoroughly cooked. Drain and return to skillet; set aside.

Whisk eggs, milk, onion powder and remaining salt and pepper in a bowl until blended.

Return beef to medium heat. Add onion and spinach; cook for about 1 minute, until spinach wilts. Stir in tomatoes. Reduce heat to low and pour in egg mixture. Cook, for about 5 minutes, using a spatula to gently lift cooked portions around the edge of pan to allow uncooked egg to flow underneath, until bottom is set and top is almost set.

Sprinkle cheese over top and place skillet under preheated broiler for 2 minutes to melt cheese and puff up frittata, or cover with lid and cook for 2 to 3 minutes on stovetop. Loosen around edge of frittata with a spatula. Cut into wedges and serve.