Chef Tip Thinly slice flank steak across the grain to keep it tender.
Heat a grill pan. Grill the jalapeńos until charred. Stem and seed the jalapeńos; finely dice 2 of them. Thinly slice the remaining jalapeńo and reserve on a platter.
In a saucepan, heat 2 tablespoons of olive oil. Add the diced jalapeńos and onion and the garlic and cook over moderate heat until golden, 10 minutes. Add the tomatillos and cook until softened. Transfer to a bowl and let cool. Stir in the avocado, cilantro and lime juice; season with salt and pepper.
Oil the chorizo and grill over moderate heat until cooked through. Remove the chorizo casing and crumble the meat into a bowl. Lightly oil the cheese and grill for 1 minute per side, then oil the pan and grill the sliced onion until charred; transfer both to the platter.
Oil the steak; season with salt and pepper. Grill over moderately high heat, turning once, 8 minutes for medium-rare. Transfer to a board and let rest for 5 minutes. Thinly slice the steak and serve with the accompaniments.