Preheat your oven to 375°F and place the chicken breast in a pot of simmering water for 12-15 minutes until cooked through, then shred with two forks.
Place the husked tomatillos, jalapeño, onion, and garlic on a baking sheet and broil for 5-7 minutes until charred and softened.
Transfer the charred vegetables to a blender with cilantro, half the lime juice, cumin, and half the sea salt; pulse until you have a smooth green sauce.
In a small oven-safe dish, spread a thin layer of the green sauce on the bottom, followed by one corn tortilla.
Top the tortilla with half of the shredded chicken and half of the black beans, then add another layer of sauce.
Place the second tortilla on top, followed by the remaining chicken and beans, and pour the rest of the sauce over the top.
Bake the enchilada stack for 15 minutes until the sauce is bubbling and the edges of the tortillas are slightly crisp.
While the enchiladas bake, mash the avocado with the remaining lime juice, sea salt, and black pepper to create a fresh guacamole.
Serve the hot enchilada bake alongside the warmed brown rice and top with the fresh guacamole.