Preheat the oven to 425F/220C and set a rack in the center of the oven. To a large bowl, add the chicken, coriander, paprika, red pepper flakes, and 1 tbsp. olive oil, ½ tsp salt, and ¼ tsp black pepper. Toss to coat the chicken in the oil and spices. Arrange the chicken slightly apart on a baking sheet.
Cut the cauliflower into florets and slice the onion into thick slices. To the same bowl, add the cauliflower and the onion together with 1 tbsp olive oil, ½ tsp salt, and ¼ tsp black pepper. Toss to coat evenly, then arrange the vegetables onto the baking sheet. Spread the vegetables in a single layer.
Transfer the baking sheet to the oven and bake for 40-50 minutes until the chicken is golden brown and cooked through and the cauliflower is slightly charred. Toss the vegetables halfway through the baking time to ensure even cooking. Meanwhile, prepare the yogurt sauce.
Add the mint to a bowl with yogurt, lemon juice, and minced garlic. Whisk to combine. Add a pinch of salt and black pepper if desired. Set aside.
Once baked, transfer the chicken and cauliflower to a serving platter. Drizzle any juices left on the sheet pan on the chicken. Optionally, squeeze some lemon on top. Serve right away.