In a large bowl, mix the ground beef with the onion, baking powder, cilantro, garlic, tomato juice or grated tomatoes, cumin, half of the olive oil, salt, and pepper. Knead very well until all ingredients are well combined and the mixture is fluffy.

Grease a round baking pan with margarine and spread the ground beef mixture.

Arrange the potatoes, peppers, and tomatoes alternately in a radial pattern on top. The thickness of the gyro should not exceed 1 1/4 inches.

Drizzle with the remaining olive oil, cover with parchment paper, and bake in the oven for 40 minutes at 356°F.

Cut the hot gyro into triangular pieces and serve with the potatoes and peppers on pita bread for souvlaki.