Position a rack near the top of the oven. Preheat the oven to 425 degrees F.
Place the chicken, cauliflower florets, and onion on a large rimmed baking sheet. Drizzle with olive oil, then sprinkle with salt, cumin, turmeric, and pepper. Toss to coat everything evenly, then spread out in an even layer, so that no cauliflower or onion is under the chicken. Place the lemons cut-side up anywhere on the baking sheet.
Place in the oven and bake for 25 minutes. Give the cauliflower and onion a stir to allow them to absorb the chicken fat, and return to the oven to cook for an additional 20 minutes. If your chicken skin isn’t golden brown and crispy, broil it for 2 to 3 minutes, watching it closely the entire time to make sure it doesn’t burn.
Working quickly while the baking sheet is hot, transfer the chicken to two plates. Squeeze the lemon juice over the cauliflower and toss in the spinach to wilt it. It helps to use tongs to do this. It won’t wilt entirely, but it will look soft when it’s ready.
Divide the cauliflower and spinach mixture between the two plates, top with feta and parsley, and enjoy immediately.