To roast chile peppers and tomatillos, place both on a baking pan coated with a little olive oil, cut side down. Roast for 35 minutes. Remove from oven and let cool. When cool, cut each 1/2 of tomatillo in 1/2 again; set aside.
Line the bottom of slow cooker with red onion slices, and 1/2 of the diced garlic, and sprinkle with salt and pepper. Lay the pork tenderloin roast on top of the vegetables. Sprinkle the remainder of the diced garlic on top of the pork. Add the roasted chiles, salsa, diced tomatoes or tomato sauce, roasted tomatillos, and additional salt and pepper. Sprinkle a little powdered cumin all over the top and stir gently to mix.
Cook on low for approximately 10 hours or until pork is tender and the meat is falling apart.
Serving suggestion: Slice or chunk pork, serve with juice, and all the vegetables on top. Mexican rice and colorful coleslaw make for a very healthy and tasty meal. This pork would also be excellent to shred and serve in tacos.