Cook the black beans. Preheat an oven to 350F with a rack in the center. Place 2 cans black beans in a single layer on a parchment-lined baking sheet. Cook for 20 minutes, or until the beans are crispy and the skins have mostly cracked. Set aside and cool slightly.
Sauté the vegetables. Meanwhile, heat 2 Tbsp oil in a skillet over medium-high heat. When hot, add 1 cup diced onion and1 cup diced poblano. Cook, stirring often, until softened, about 5 minutes. Add 3 cloves of chopped garlic, then cook 1 minute more. Stir in the chopped chipotle chili and 1 tsp adobo sauce, then cook for about 30 seconds. Transfer mixture to a large bowl.
Grind the cashews. Place ¾ cup cashews in the bowl of a food processor fitted with blade attachment. Pulse until finely chopped, then add to the bowl with onions and peppers.
Pulse the beans. Next, place the cooled beans in the food processor along with ½ cup feta. Pulse until the beans are roughly chopped, then transfer to the bowl with the onion/pepper mixture. Add 2 Tbsp mayonnaise, 1 egg, and ¾ cup panko. Season with 1 tsp Kosher salt and ¼ tsp pepper. Mix everything together with your hands. At this point, you can refrigerate the mixture for up to 3 days.
Cook the black bean burgers. Form the mixture into 6-8 equal sized patties, as wide as your burger buns. If grilling, preheat a grill over medium-high heat. Spray the grates with grill spray, then cook for 3-5 minutes per side over direct heat. If cooking indoors, preheat a large nonstick or cast iron skillet over medium to medium-high heat. Coat with 1 Tbsp olive oil, then cook patties, working in batches as needed, for 4-5 minutes per side.
Assemble, then serve. Spread buns with any condiments, then add toppings to top or bottom bun as desired. Place a black bean burger on top, then serve immediately!