Combine coriander, fennel, allspice and black pepper. Reserve. Combine 2 cups kosher salt and 1 cup brown sugar. Reserve.
Place pork belly in nonreactive casserole dish. Spread evenly with Florida orange zest and thyme. Sprinkle with Florida orange and tangerine juices. Season with spice mixture. Rub kosher salt and brown sugar mixture evenly over pork belly. Cover and refrigerate for 5 hours.
While pork belly is curing, prepare Maple-Bourbon Glaze: Combine 1 cup Florida orange juice, bourbon, maple syrup, white wine vinegar and orange marmalade. Bring to simmer in medium saucepot and reduce volume by ⅔. Add butter and stir until melted. Season to taste with salt and pepper. Pour into blender and blend until smooth. Reserve.
To prepare Grapefruit Slaw: Place napa and red cabbage in mixing bowl. Sprinkle with kosher salt and let stand for 10 minutes. Use a piece of cheesecloth to squeeze liquid from cabbage. Discard liquid. Add Florida grapefruit segments to cabbage. Combine Florida grapefruit zest, olive oil mayonnaise, sugar and lime juice. Pour over shredded cabbage and Florida grapefruit segments and toss to combine. Reserve.
Remove pork belly from refrigerator and rinse under cold water to remove salt mixture. Place in ovenproof casserole dish with lid and bake at 300F for 1½ hours. Remove from oven and allow to cool to room temperature. Slice into ½” slices. Sauté in large skillet to brown and crisp both sides of pork belly slices. Remove to plate lined with paper towels to remove any excess oil.
To serve, place 1 slice of pork belly on each plate and drizzle with Maple-Bourbon Glaze. Serve with Grapefruit Slaw on the side.