Heat oil in a large skillet over medium-high heat. Cook onion, garlic, and salt until onion begins to soften, 5 to 7 minutes.
Add lentils and chili powder, cumin, and paprika. Cook until about 1 minute. Add chipotle chile and broth; bring to a boil.
Reduce heat, cover, and simmer until lentils are tender, 25 to 30 minutes.
Uncover lentils and cook until mixture thickens, 6 to 8 minutes. Spoon about ¼ cup lentil mixture into each taco shell. Top with lettuce, tomato and cheese.
Serve with salsa, sour cream or yogurt, and/or avocado if desired.