Prep the chicken thighs. Cut meat into cubes, and season chicken with salt. Let rest on the counter for 15-20 minutes while you make the salsa verde.
Roast salsa verde veggies. Heat a griddle, comal or skillet on medium heat and roast whole tomatillos, onion, garlic, serrano and poblano, turning frequently to avoid burning things. Note, a little char is okay and will add flavor to your final sauce. The onion and the garlic will cook faster than everything else, so remove from heat when they are ready. If peppers and tomatillos have a nice char on them but are still not done cooking, lower heat and cover, checking and turning until you are satisfied with doneness. (My tomatillos changed color even in the center and one exploded on me when I picked it up with a pair of cooking tongs.)
Make nopales. Heat a frying pan over medium heat. Slice and cut nopales. Add a glug of olive oil (1-2 tablespoons depending on whether or not the pan is non-stick or not). Once hot, add nopales pieces, salt, garlic, and onion. Saute until nopales fade from bright green and most of the baba, or juices, have been released and evaporated, stirring occasionally. Reduce heat and cover if nopales start to brown, still stirring occasionally. The whole process will take 10-15 minutes. Turn off burner once nopales are cooked through and set aside.
Make salsa verde. Peel garlic. Rip tops off of peppers. Deseed/devein serrano pepper(s). Add all of the salsa verde ingredients except the serrano peppers to a blender or a food processor and puree until you have a smooth sauce. Taste. Add serrano peppers in increments, pureeing and tasting until you achieve your desired heat level.
Assemble the dish. In a large pot, add another glug of olive oil (approximately 2 tbsp) and use medium heat to saute onions for 1-2 minutes or until they start to turn translucent. Add garlic and chicken. Saute for 5 minutes or until the chicken starts to brown. Add salsa verde and nopales. Simmer for 15 minutes or until chicken is cooked through.
Buen provecho! Serve over a bed of white rice, garnished with fresh cilantro, with warm tortillas and freshly sliced avocado.