If using homemade, make taco seasoning according to your recipe.
Cook the quinoa: in a small saucepan combine 1 cup rinsed quinoa, 1 Tbsp taco seasoning, and 1 1/2 cups water or vegetable broth; bring to a light boil over medium-high heat.
Reduce heat to medium-low, cover, and simmer until liquid is absorbed, about 15–20 minutes; remove from heat and fluff with a fork.
While quinoa cooks, thinly slice the onion and bell peppers and mince the garlic if using.
Heat a large skillet over medium-high and add the sliced onion and bell peppers; sauté with 2 Tbsp taco seasoning for 8–10 minutes until softened, adding a splash of vegetable broth or water as needed for a no-oil method.
Combine filling: transfer cooked quinoa to a large bowl, then stir in the rinsed black beans and drained diced tomatoes until evenly mixed.
Assemble burritos: lay a tortilla flat, spoon some quinoa-bean mixture down the center, top with sautéed onion and peppers and about 1/2 cup salsa, then add any desired toppings.
Roll the tortilla by folding in the sides and then rolling from one end to the other; if desired, crisp the burrito in a hot skillet or grill pan for a few minutes to finish.