Heat grill: Heat the grill to 375-400F degrees with a direct heat zone.
Prep: Slice 2 tangerines. Zest and juice the other tangerine.
Sauté: Place a small sauce pot on the grill, and melt one tablespoon of butter in it. Add the shallots and garlic and sauté for 1 minute.
Make Butter Sauce: Add 2 chopped sage leaves, 1/2 teaspoon salt, 1/2 teaspoon pepper, red pepper flakes, the tangerine juice, the remaining butter and half of the tangerine zest. Stir until melted and combined. Remove the pot from the grill.
Season: Season the halibut with 1/2 teaspoon salt and 1/2 teaspoon pepper on both sides.
Grill: Arrange the tangerine slices on the grill in two overlapping rows the length of your fish filet. Place the fish on top of the orange slices. Spoon some of the tangerine butter sauce on top. Close the lid and grill for 10 minutes.
Sauce: Open the grill lid. Spoon more sauce on top of the fish. Set the remaining sage leaves on the grill off to the side. Close the lid and grill for 5 more minutes.
Serve: Remove the halibut to a platter. Drizzle the remaining sauce on top. Sprinkle with remaining tangerine zest and crispy sage leaves.