Heat oil in a skillet over medium heat; cook and stir onion and garlic until tender, about 5 minutes. Mix lentils and taco seasoning into onion mixture; cook and stir for 1 minute.
Pour chicken broth into skillet and bring to a boil. Reduce heat to low, cover the skillet, and simmer until lentils are tender, 25 to 30 minutes.
Uncover the skillet and cook until mixture is slightly thickened, 6 to 8 minutes. Mash lentils slightly; stir in salsa.
Serve about 1/4 cup lentil mixture in each taco shell.
