Turn your broiler to high. Place 4 garlics, poblano pepper, serrano chile and tomatillo in a baking sheet lined with aluminum foil and place it in the oven and broil until they are lightly charred and turn them once. Place them in a large mixing bowl and cover with plastic wrap and let it stand for 10 minutes. Uncover, and peel off the skin of the peppers and tomatillo. Put them into the food processor and add in the rest of the ingredients for the green chile and pulse to mix everything. Set aside. Wash the food processor to get ready to prepare the black bean patty
Place the black beans, mushrooms and garlic in a food processor and pulse until coarsely chopped. Add in the bread crumbs, cumin salt, and egg and pulse again until mix. Transfer to a large mixing bowl. Divide the black bean mixture into 6 equal portion and shape with your wet hands into around 1/2-inch thick patties
Heat a large non-stick skillet (with cover) over medium heat. Add in about 1 Tbsp of cooking oil and swirl to pan to coat. Add in the patties and cook for about 10 minutes and then flip to the other side. Place one slice of cheese on the patty and cook for another 2 minutes. Cover the skillet and cook until the cheese melts
Assembling the burgers
Place a small handful of baby spinach leaves on the bottom half of each bun. Top with the black bean patty and followed by one red onion ring. Smothered some of the green chile on top of this and top with the other halves of the buns and serve immediately