In a large skillet over medium-high heat, cook the beef and onion for 6–7 minutes, breaking up the meat, until browned. Drain excess fat.
Add the garlic and taco seasoning and cook 1 minute.
Stir in the lentils, water or broth, and tomato sauce. Bring to a boil.
Reduce the heat, cover, and simmer 20–25 minutes, stirring occasionally, until the lentils are tender and the liquid is mostly absorbed. Add a splash of water if it gets too dry.
Mash lightly with a spoon to blend the lentils into the beef. Taste and adjust seasoning.
Spoon into warm tortillas and top as desired.