Prepare Blood Orange Zest and Slices: Grate the zest of one blood orange into a large bowl. Slice the ends off the oranges, peel away the pith, and slice crosswise into ¼-inch-thick rounds. Set aside.
Mix Marinade: To the bowl with the zest, add Scotch whiskey, brown sugar, mustard, lemon juice, fennel seeds, thyme, garlic, salt, pepper, and chile flakes. Stir well.
Marinate Chicken and Vegetables: Halve the red onion and fennel bulb lengthwise. Slice into ¼-inch wedges, keeping the root intact. Add onion, fennel, and chicken pieces to the marinade. Toss thoroughly and let stand at room temperature for 30 minutes (or refrigerate overnight).
Prepare Oven: Position an oven rack in the center and preheat to 475°F (245°C).
Roast Chicken and Vegetables: Remove chicken and vegetables from the marinade, discarding excess liquid. Arrange in a single layer on a baking sheet. Drizzle with olive oil. Roast for about 30 minutes until the chicken is cooked through.
Broil for Final Touch: Turn the oven to broil. Broil chicken and vegetables for 1–2 minutes until the skin is blistered. Watch carefully to prevent burning. Transfer to a platter and tent with foil to rest.
Caramelize Oranges: Arrange reserved orange slices in a roasting pan. Sprinkle with granulated sugar and broil for 5–10 minutes until caramelized.
Serve: Top the chicken and vegetables with caramelized oranges, garnish with fennel fronds, and serve.