Preheat the oven to 400°F. Arrange taco shells in a 9x13-inch baking dish and set aside.
Cook quinoa according to package instructions, then drain any excess water and set it aside.
In a large skillet, heat the oil over medium-high heat. Add the onion and cook for 3 minutes, until softened.
Add the cooked quinoa, drained black beans, taco seasoning, lime juice, salt, and pepper to the skillet. Stir to combine, then add the water to keep everything moist. Reduce heat to medium and cook for 3-5 minutes, until heated through. Adjust seasoning to taste.
Spoon the black bean mixture into the taco shells. Bake for 3-5 minutes in the preheated oven until the shells are warm and slightly crisp. Add desired toppings and serve immediately.