Place chicken on one of the prepared pans and sprinkle with 1/4 teaspoon salt and pepper. Spread tomatillos, onions, jalapeños and garlic on the other pan. Transfer both pans to the oven. Roast the chicken, flipping once, until an instant-read thermometer inserted in the thickest part registers 165 degrees F, 15 to 25 minutes. Roast the vegetables, stirring occasionally, until tender and browned, about 25 minutes. Reduce oven temperature to 375 degrees.