In the recipe in the book, Sodha has you mash the ginger, garlic, chiles and spice in a mortar and pestle to a paste. I suppose you could also process them in a food processor or blender. I went for the method that required the least extra objects and just minced everything and mixed it, which is how I’ve written it below. If you’d prefer not to mince, definitely use one of these other methods.
I show this (perhaps confusingly) in a 10×15-inch pan, which is the biggest that fits in my tiny oven. I scaled the recipe for a proper sheet (technically a half-sheet) pan most of us use, a 13×18-inch. You could also divide this over 2 quarter-sheet (9×13-inch) pans. The 10×15-inch version you see here yielded an awkward three servings, fine for us, not for most people.
Cooking times will of course vary by the size of your chicken pieces. When roasting chicken breasts (always large) and drumsticks and thighs together, I halve the breasts right through the bone to even up the cooking time.
Don’t have garam masala? Sadly, neither do I after purging some dusty spices all of five days ago. (D’oh!) To roughly approximate 1 1/3 teaspoons (yes, slightly more than you’ll need because I’m presuming everyone will be missing a spice or two below) garam masala you can use: 1/2 teaspoon ground turmeric, 1/4 teaspoon ground coriander, 1/8 teaspoon each ground cumin, cardamom and mustard, 1/16 teaspoon each ground black pepper and ground fennel, plus a pinch of ground cloves and ground cayenne. “Deb, you’ve got to be kidding me…” I know, I know. You could also just add a few extra pinches of several of those and have a delightfully spiced outcome.
Worried about spiciness? Because I think this is a good thing to know about whoever is writing your recipes, I consider myself a moderate heat appreciator, but I cook fairly mild because my 6.5 year old is not. Even with a full jalapeno in there (which can run quite mild or hot, so not necessarily a perfect measure) and a 1/4 teaspoon spicy chili powder, this was just lightly hot. The yogurt really dulls the heat. Hope that offers a guidance so you can adjust it to your preference.