Whisk onion, lemon (or lime) juice, oil, garam masala and salt in a bowl until well combined.
Place chicken in a shallow dish or 1-gallon sealable plastic bag. Add the marinade and refrigerate for at least 1 hour and up to 12 hours. Remove from the marinade and pat dry.
Preheat grill to medium-high or position a rack in upper third of oven and preheat broiler.
To grill: Oil the grill rack (see Tip). Grill the chicken, turning once, until an instant-read thermometer inserted into the thickest part registers 165 degrees F, 4 to 8 minutes per side.
To broil: Line a broiler pan (or baking sheet) with foil and coat with cooking spray. Place the chicken on the foil. Broil, watching carefully and turning at least once, until an instant-read thermometer inserted into the thickest part registers 165 degrees F, 10 to 15 minutes total.
Make Ahead Tip: Cover and refrigerate the marinade for up to 3 days; marinate the chicken for up to 12 hours.
Garam masala, a blend of spices used in Indian cooking, usually includes cardamom, black pepper, cloves, nutmeg, fennel, cumin and coriander. It is available in the spice section of most supermarkets.
It’s difficult to find an individual chicken breast small enough for one portion. Removing the thin strip of meat from the underside of a 5-ounce breast–the chicken tender–removes about 1 ounce of meat and yields a perfect 4-ounce portion. Wrap and freeze the tenders and when you have gathered enough, use them in a stir-fry or for oven-baked chicken fingers. If you can only find chicken breasts closer to 8- to 9-ounce each, you’ll only need 2 breasts for 4 servings–cut each one in half before cooking.
Tip: Oiling a grill rack before you grill foods helps ensure that the food won’t stick. Oil a folded paper towel, hold it with tongs and rub it over the rack. (Do not use cooking spray on a hot grill.) When grilling delicate foods like tofu and fish, it is helpful to coat the food with cooking spray.