This marinade is best on quick-cooking cuts of meat like this spicy grilled chicken, as the sugars in it can burn if on the grill for too long. Use the grill to cook the rest of your meal, too--Japanese eggplant and shishito peppers would be nice.
Whisk tamarind sauce, garlic, ginger and garam masala in a 9-inch baking dish. Add chicken and turn to coat. Marinate at room temperature, turning once, for 30 minutes.
Preheat grill to medium-high.
Remove the chicken from the marinade (discard the marinade). Oil the grill rack. Grill the chicken, turning once halfway, until an instant-read thermometer inserted in the thickest part reaches 160 degrees F, 6 to 10 minutes. Let rest for 5 minutes. Sprinkle with scallions and serve with more sauce, if desired.
Spicy Tamarind Sauce