Preheat the oven to 350°F. Line a baking sheet with parchment paper and set aside.
Heat the oil over medium heat in a skillet. Add the diced onions and sauté for 4 to 5 minutes until soft and translucent. Add the garlic and sauté 1 more minute. , chili powder, cumin, smoked paprika, and oregano and sauté 1 to 2 more minutes until fragrant and all of the liquid is absorbed. Let cool.
Meanwhile, in the bowl of a food processor, pulse the pumpkin seeds until they resemble coarse sand.
Add to the food processor with the pumpkin seeds, the chili powder, cumin, smoked paprika, oregano, diced green chiles, mayonnaise and sautéed onions and garic. Pulse until combined, but NOT puréed.
Add the black beans, quinoa, and flax meal and pulse again until just combined.
The mixture should hold together well and not crumble when pressed with your fingers. If the mixture seems too wet, add another tablespoon or two of flax meal.
Using your clean hands, form the mixture into 4 burger patties.
Place the patties on a plate in the fridge for 20 to 30 minutes to firm up. This step is optional, but helps to create a sturdier burger.
Bake the burger patties on the prepared baking sheet for 25 to 30 minutes, carefully flipping after 15 minutes.
While the burgers are baking, make the Spicy Mayo, if using. Whisk all ingredients in a small bowl until smooth. Taste and adjust seasoning as needed.
Serve on burger buns with the Spicy Mayo, if using, and any toppings of choice.