Sauté onion, garlic, and peppers in oil until onion is transparent (about 5 minutes). Add turkey and chili powder and cook over medium heat about 5 or 6 minutes, stirring and separating turkey as it cooks. Stir in tomato salsa and tomato paste and simmer for 5 minutes.
Serve in taco shells, topped with shredded cheese and other garnishes, if desired.