Preheat the broiler to high and adjust an oven rack to the top level.
Whisk together the lime juice, olive oil, garlic and a pinch of salt and pepper in a medium bowl.
Dice the peppers and onion into bite-size pieces (about 1/2 inch).
Place the tomatillos stem-side down on a sheet pan and place under the broiler. Char until the tops are blackened, about 3 minutes. Flip and broil 3 more minutes. Set aside to cool.
Meanwhile, heat a large cast-iron skillet on high heat and add the peppers and onions. Spread them out in an even layer and sear in the dry skillet until lightly charred, about 4 minutes.
Dice the tomatillos into 1/2-inch chunks. Toss all the vegetables into the bowl with the lime dressing. Stir in the diced avocado and top with the cilantro, queso fresco and toasted pepitas.