First, in a large frying pan over medium-high heat, sweat the onion in 1 tbsp. oil. Stir frequently, about 4-5 minutes until the onion is translucent. Then, add the garlic and spice mixture and cook for another minute or two, stirring frequently. Remember, you don't want to burn the spices just crank up the flavor by cooking them.
Next, add the lentils and stir to combine. Then, add the vegetable broth and using a potato masher or fork, gently mash the lentils until they start to look like ground meat. Then, cook another few minutes until hot. Note, if the lentils start to dry out, add another tbsp. or two of vegetable broth (can also use salsa here)
While you are cooking this, warm the tortillas. You can do this in the oven or microwave. I just place them on the burner behind my frying pan and let them warm up while I cook the filling, this way I don't have to turn on the oven.
Finally, place a heaping spoonful or two into the taco shell and add your fillings. The filling will make about 8 tacos, depending on how stuffed you like them!
Notes
If you are using dried lentils, you'll need to cook them first. Rinse the lentils, then place in a saucepan with enough water to cover by 2-3 inches. Bring to a boil, then reduce heat to medium and let simmer until tender, about 25-30 minutes.
Instead of the homemade spice mix, you can use a packaged taco seasoning. Start with 2 tablespoons, then add more as desired.