Place racks in middle and upper third of oven; preheat to 425°. Place half of a loaf sourdough bread (about 10 oz.), torn into 2" pieces, on a large rimmed baking sheet and drizzle with ¼ cup extra-virgin olive oil; season with kosher salt. Toss until bread is coated and has absorbed all the oil. Spread out in an even layer and bake on middle rack, tossing halfway through, until golden brown and crisp, 12–15 minutes. Set croutons aside; keep oven on.

Meanwhile, whisk together 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 ground pepper. Add 4 skin-on, bone-in chicken thighs (about 1½ lb.), patted dry, and turn to coat. Arrange thighs on another large rimmed baking sheet, spacing evenly apart; reserve bowl (do not rinse out).

Combine 1 large fennel bulb, fronds reserved, cored, cut through root end into 1"-thick wedges, and 1 lb. seedless red grapes in reserved bowl. Drizzle remaining 1 Tbsp. extra-virgin olive oil over and season with salt; toss to coat. Arrange fennel and grapes on baking sheet with chicken, filling in gaps. Roast on middle rack until grapes are slightly softened and chicken is cooked through (an instant-read thermometer inserted into the thickest part of a thigh without touching the bone should register 165°), 25–35 minutes, depending on size.

Carefully tilt baking sheet to pour off any juices over reserved croutons. Drizzle 2 Tbsp. Champagne vinegar or white wine vinegar over croutons; toss to soak up liquid. Arrange croutons on baking sheet with chicken; pour any remaining juices back over croutons.

Heat broiler. Transfer baking sheet to upper rack and broil until chicken skin is browned and crisp and edges of fennel and croutons are dark brown, 1–2 minutes. Remove from oven and scatter ¹⁄₃ cup Castelvetrano olives, pitted, torn, and some reserved fennel fronds over.