Heat a large skillet over medium heat, then add oil and 1/2 of the chopped onion {~ 1/2 cup} to the pan, stirring until onion is softened, ~ 2 minutes. Add cooked lentils to the pan and stir, then add cumin, chili powder, salt, pepper, and water to lentil/onion mixture, and stir well to combine. Turn heat down to low to simmer while preparing the avocado feta guacamole.

Remove skin and pit from avocados and place them in a medium sized mixing bowl. Mash avocados with a fork, then add remaining chopped onion and feta cheese, stirring well to combine.

Spoon lentil "meat" mixture in to a serving bowl, and place it on the table along with the taco shells, avocado feta guacamole, and desired taco fixins' so everyone can serve themselves "Taco Bar Style".