For the zucchini taco shells: Preheat the oven to 450 degrees F.
Grate the zucchini with a box grater or the shredder attachment on a food processor; after doing so, you should have about 4 cups. Place the zucchini in a large microwave-safe bowl and microwave on high, stirring after 3 1/2 minutes, until tender and liquid has collected in the bottom of the bowl, about 7 minutes. Set the zucchini aside until it is cool enough to handle.
Once cool, use a slotted spoon (or your hands) to transfer it to a kitchen towel or piece of cheesecloth; roll up the cloth and twist and squeeze out as much liquid as you can. Squeeze hard to get out all the excess water; this is crucial to getting good taco shells. You should end up with about 2 cups of shredded cooked zucchini.
Line a baking sheet with parchment paper and spray it with cooking oil to help prevent sticking. Add the zucchini, Parmesan, scallions, chili powder, cumin, garlic powder, salt and eggs to a medium bowl; stir until blended into a dough. Scoop this mixture into 1/4-cup-size balls and place them on the prepared baking sheet. Use your hand to flatten each ball into a very thin 5-inch diameter round, about 1/8 inch thick. Bake the shells until they are brown around the edges, about 15 minutes. Let them cool and then remove them from the parchment.
For the zucchini-tomato salsa: Add the tomatoes, basil, mint, salt, zucchini and lime juice to a medium bowl and toss together.
For the taco filling and fixings: Heat the oil in a large skillet over medium heat. Add the ground beef, shallot, serrano and 1/2 teaspoon of the salt and cook this mixture until the beef is done through and the veggies are tender, 3 to 5 minutes. Add the corn kernels, ancho chile powder, cumin, 1/4 cup water and the remaining 1/2 teaspoon salt. Cook this mixture for 3 minutes more, adding more water if needed to thin the taco filling out.
Spread about 1/4 cup of the refried beans onto each zucchini taco shell. Place 1/4 cup of the beef filling onto each, then top with 1 heaping tablespoon of the zucchini tomato salsa and 1 heaping teaspoon of the feta. Fold up and enjoy!