Heat the oven to 425°F and set a rack in the upper middle position. Line a large rimmed sheet pan with parchment or lightly oil the pan so the yogurt coating does not stick.

Pat the chicken thighs dry with paper towels and set them on a plate. Dry surfaces help the coating cling better and brown more evenly.

In a large bowl, stir together the yogurt, garam masala, paprika, 1 tablespoon of the oil, 1 teaspoon of the salt, and 1 tablespoon lemon juice until smooth and evenly colored.

Put the chicken thighs into the bowl of yogurt mixture. Turn the pieces until they are fully coated, then set the bowl aside while you finish the vegetables.

Put the cauliflower florets and red onion wedges onto the sheet pan. Keep them in a single loose layer so the hot air can reach them.

Drizzle the vegetables with the remaining 1 tablespoon oil. Sprinkle the remaining 1/2 teaspoon salt over them.

Use your hands or a spatula to toss the cauliflower and onion right on the sheet pan. Spread them back out so they are not piled up.

Make open spaces on the sheet pan for the chicken. This keeps the vegetables from steaming under the meat.

Lift the chicken thighs from the bowl and place them on the sheet pan among the vegetables. Scrape any extra yogurt mixture from the bowl onto the tops of the chicken pieces.

Slide the sheet pan into the oven and roast for 15 minutes. The yogurt coating will start to dry on the surface and the onions will begin to soften.

Pull the sheet pan out of the oven and set it on a heat-safe surface. Use a spatula to turn the cauliflower and onion so they brown on another side.

If any onion wedges are getting darker than the rest, tuck them closer to the center of the pan. Leave the chicken in place so the coating stays on top.