Heat oil in a large sauté pan over medium-high heat. When the oil warms, add onion and cook until onion becomes translucent. Add ground meat and cook, breaking it up with the side of a spoon, just until meat is cooked through, 3 to 4 minutes.
Stir in taco seasoning and tomato paste and stir to combine. Add broth or water and simmer, stirring occasionally, until liquid becomes thick and clings to the meat.
Divide meat mixture, lettuce, and cheese among taco shells and serve immediately with optional toppings.