Step 1
Place racks in center and higher third of oven; preheat to 425°. Place half of a loaf sourdough bread (about 10 oz.), torn into 2″ items, on a big rimmed baking sheet and drizzle with ¼ cup extra-virgin olive oil; season with kosher salt. Toss till bread is coated and has absorbed all of the oil. Unfold out in a good layer and bake on center rack, tossing midway by means of, till golden brown and crisp, 12–quarter-hour. Set croutons apart; preserve oven on.
Step 2
In the meantime, whisk collectively 3 garlic cloves, finely grated, ¼ cup extra-virgin olive oil, 2½ tsp. smoked paprika, and 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt in a medium bowl; season with freshly floor pepper. Add 4 skin-on, bone-in rooster thighs (about 1½ lb.), patted dry, and switch to coat. Organize thighs on one other massive rimmed baking sheet, spacing evenly aside; reserve bowl (don’t rinse out).
Step 3
Mix 1 massive fennel bulb, fronds reserved, cored, minimize by means of root finish into 1″-thick wedges, and 1 lb. seedless crimson grapes in reserved bowl. Drizzle remaining 1 Tbsp. extra-virgin olive oil over and season with salt; toss to coat. Organize fennel and grapes on baking sheet with rooster, filling in gaps. Roast on center rack till grapes are barely softened and rooster is cooked by means of (an instant-read thermometer inserted into the thickest a part of a thigh with out touching the bone ought to register 165°), 25–35 minutes, relying on measurement.
Step 4
Fastidiously tilt baking sheet to pour off any juices over reserved croutons. Drizzle 2 Tbsp. Champagne vinegar or white wine vinegar over croutons; toss to absorb liquid. Organize croutons on baking sheet with rooster; pour any remaining juices again over croutons.
Step 5
Warmth broiler. Switch baking sheet to higher rack and broil till rooster pores and skin is browned and crisp and edges of fennel and croutons are darkish brown, 1–2 minutes. Take away from oven and scatter ¹⁄₃ cup Castelvetrano olives, pitted, torn, and a few reserved fennel fronds over.
