Preheat the oven to 425°. On a large rimmed baking sheet, drizzle the cauliflower, blood oranges, and rosemary with 2 tablespoons oil; season with salt and pepper. Toss to coat, and spread into an even layer.
Season the chicken thighs with salt and pepper, and nestle into the cauliflower mixture, skin side up. Drizzle skin of chicken with 1 tablespoon oil, and sprinkle with sumac. Roast until cauliflower is golden and chicken is cooked through, about 45 minutes.
Meanwhile, in a small bowl, whisk together the remaining 2 tablespoons oil, the vinegar, and blood orange juice. Season with salt and pepper.
Drizzle roasted chicken and cauliflower with vinaigrette. Scatter feta, mint, olives, and jalapeño over top, and serve with pan drippings drizzled over top.
Tangy, citrusy sumac heightens the sweetness of blood oranges.Recipe by Laura Rege.