1/4 cup fennel seeds
2 teaspoons lemon zest
2 teaspoons minced garlic plus 1 head of garlic, halved crosswise
4 teaspoons kosher salt plus more for seasoning
1 teaspoon freshly ground black pepper plus more for seasoning
1 teaspoon crushed red pepper flakes
4 fennel bulbs (3 pounds), trimmed, each cut into 8 wedges, plus 1/4 cup fronds
2 pounds fingerling potatoes, halved lengthwise
24 small carrots, peeled or scrubbed
8 shallots, halved lengthwise
3 lemons, 1 thinly sliced, 2 halved
1 orange, cut crosswise into 8 slices
8 tablespoons olive oil, divided, plus more
2 4-pound kosher chickens