1. Pat chicken dry and season with salt and pepper. Brush onion rounds and jalapeño with oil and season with salt. Grill chicken about 5 min per side, grill onion and jalapeño until lightly charred and soft. Transfer to cutting board and tent with foil. Grill tortillas 1-2 min per side.
2. Pulse onion, jalapeño, tomatillos, cilantro, lime juice, garlic and 1/2 tsp salt in food processor. Slice chicken and serve with tortillas, salsa and cheese.