Heat oil in a large saute pan over medium-high heat. When oil shimmers, add onion and cook until onion becomes translucent. Add turkey and cook, breaking up meat with the side of a spoon, just until meat is cooked through, 3 to 4 minutes.
Stir in chili powder, salt, and tomato paste and cook 1 minute. Add broth and simmer, stirring occasionally, until liquid becomes thick and clings to meat.
Divide meat mixture, cheese, lettuce, and tomato among taco shells and serve immediately with optional sour cream, pico de gallo, and avocado.