Sheet-pan dinners don't have to be boring! Marinating chicken in creamy, tangy spiced yogurt gets you a juicy and super-tender result, and since you're roasting your veggies on the same sheet pan as the meat, they're going to pick up all of those delicious chicken-y flavors. All-in-one dinners like this run the risk of being kind of one-note, which is why that spicy, acidic, herby finishing condiment is key—it lends contrast, and brings the whole thing together.
Place a rack in upper third of oven; preheat to 425°. Finely grate 3" piece ginger into a medium bowl (don’t worry about peeling it). Transfer all but 1 tsp. grated ginger to a large resealable plastic bag, reserving ginger in bowl.
Add 2 cups yogurt to bag with ginger. Cut 1 lemon in half crosswise and squeeze juice into yogurt mixture. Add ¼ tsp. cayenne and 2 Tbsp. plus 1 tsp. garam masala. Seal bag and shake to combine.
Season 8 chicken pieces with 5 tsp. salt. Place in bag, seal, and shake again to cover all the pieces. Squeeze out as much air as you can from the bag, which makes sure that all the chicken is covered with the marinade—this is why marinating in a resealable plastic bag is so clutch. Let marinate at least 30 minutes at room temperature, or keep in the fridge for up to 4 hours.
Meanwhile, prep your cauliflower. Cut 1 head of cauliflower into quarters through stem end. Place quarters cut side down and remove most of the core. The cauliflower florets should fall apart and separate naturally, but you can slice any larger florets in half or into quarters until all florets are about 2" in size. You want the more or less evenly sized so they're cook evenly.
Transfer cauliflower to a large foil-lined rimmed baking sheet. (That foil makes for easier cleanup later.) Drizzle 3 Tbsp. oil and 1½ tsp. salt over and toss to coat. Spread cauliflower evenly all the way to edges of pan, making sure that the pieces aren't too crowded, otherwise they'll steam instead of roasting—if you ended up with more cauliflower than can fit in a single layer, take some out and save it for something else.
Remove chicken from marinade, slightly shaking off excess. Arrange on baking sheet skin side up, nestling into cauliflower. It’s okay if some marinade clings to the chicken, just make sure it’s not pooling in pan.
Roast chicken and cauliflower, turning pan halfway through and moving chicken around with tongs to ensure even browning, until chicken skin is blistered, meat is tender all the way to the bone, and cauliflower is nicely charred, 35–40 minutes.
Meanwhile prepare your condiment. Finely chop ½ onion, place it in a colander, and rinse it with cold water. (This removes some of the onion's sharpness.) Add onion to the bowl with the reserved ginger. Finely grate zest of remaining lemon into bowl. Cut lemon in half and squeeze 1 half into bowl; reserve remaining half for another use. Thinly slice 1 jalapeño into rings and add to bowl. Coarsely chop ½ bunch cilantro (stems and all, they're tender and delicious!) and add to bowl. Season with remaining 2 tsp. garam masala and ¾ tsp. Mix with a spoon to combine, taste, and add more salt and lemon juice if necessary—you want the condiment to be plenty zippy.
Arrange chicken and cauliflower on a platter. Top with lemon condiment and serve with remaining ½ cup yogurt alongside for drizzling.