For the roasted salsa: Preheat the broiler.
Lay out the tomatillos, serranos, the onion half and the garlic on a baking sheet. Broil for 20 minutes, until beginning to char. Flip and broil on the reverse side until charred; remove each item as it chars, until all are done. (Keep a close eye¿you don't want the vegetables overcharred). Set aside to cool to room temperature.
In a food processor or blender, combine the roasted vegetables with the spinach, cilantro and lime juice. Puree until smooth, adding 2 tablespoons water as needed. Season with salt.
For the avocado crema: In a blender or food processor, combine the avocado, Greek yogurt, lime zest and juice, salt and cumin. Process until smooth. If needed, add 5 to 6 tablespoons water to the puree; it should be like soft-serve ice cream.
For the burrito: Heat the olive oil in a 12-inch skillet over medium-high heat. When it starts to smoke slightly, add the sweet potatoes and cook, without stirring, until well browned, 6 to 7 minutes. Once browned, stir and continue to brown and crisp until cooked through, 4 minutes more. Sprinkle with the salt, cayenne and ancho, then remove from the pan and set aside.
Lower the heat to medium and add the crumbled chorizo to the skillet. Cook until the fat has rendered, 2 1/2 minutes. Add the sliced scallions and garlic to the pan. Cook until fragrant, 30 seconds. Add the pinto beans and orange juice; let sizzle and reduce for about 1 minute, until it just coats the bottom of the pan. Remove from heat and stir in the cilantro.
To build the burrito: Lay a tortilla on a work surface. Fill the rectangular area just below the middle of the tortilla and about 1 1/2 inches from either side with 1/4 cup of the sweet potato, 1/4 cup of the bean mixture, 2 tablespoons of the salsa and 1 tablespoon of the avocado crema. Top with one-quarter of the alfalfa sprouts.
For each tortilla, fold in the sides and then fold up the bottom end, being careful to pull the sides in. Tuck the bottom flap close to the filling and roll forward to close the burrito.
Preheat a griddle and a cast-iron pan over medium-high heat. Add 1 ounce of the cheddar to the griddle in a strip. Lay a burrito seam-side down on top of the cheese. Weight the burrito with the hot cast-iron pan and cook until the tortilla browns and the cheese releases, 30 seconds to 1 minute. Flip the burrito over and brown on the other side. Remove from the heat, and repeat with the remaining burritos. Slice the burritos in half and serve.