How to Make It
Preheat the oven to 400°. On a rimmed baking sheet, toss the fennel, 1/4 cup of oil, the thyme and crushed red pepper. Season with salt and black pepper. Roast for about 20 minutes, until just tender.
Gently curl 1 snapper fillet over the edge of a ceramic baking dish, skin side up. Make 5 or 6 slashes in the fillet. Repeat with the remaining fillet.
In a bowl, whisk the remaining 1 1/2 tablespoons of oil, the fennel seeds and zest and rub all over the fillets and in the slashes; season with salt and pepper. Arrange the snapper skin side up on the fennel. Roast for about 8 minutes, until the fish is just cooked through. Serve the fish and fennel with the grapefruit sections.