If you have a grill basket, prepare a grill to medium-high to high heat. Or, preheat broiler to high and set a rack in the upper third of the oven.
Arrange the tomatillos, onions, garlic and peppers in the grill basket (if grilling) or on a baking sheet (if broiling), spray lightly with olive oil spray and season with salt and pepper. Cook veggies until charred.