Preheat the oven to 400 degrees F. On a large rimmed baking sheet, toss the polenta with the cauliflower, onion, and ¼ cup of olive oil. Brush the chicken legs with oil and season with salt and pepper. Nestle the chicken into the polenta and vegetables. If needed, divide between 2 baking sheets.
Roast for 50 minutes, until golden and an instant-read thermometer inserted in the thighs registers 165 degrees F. Drizzle with lemon juice, garnish with parsley, and serve with lemon wedges.