1. Rinse fish and pat dry. Whisk together honey, 5 tablespoons juice, the pepper flakes, garlic, and oil in a glass dish just large enough to fit fish in a single layer. Arrange fish in dish, turning to coat, and marinate at room temperature, turning halfway through, 20 minutes.
2. Meanwhile, peel mango and from the two flatter sides, starting from the outside moving in toward pit, cut into 1/8-inch-thick slices. Cut into julienne strips and transfer to a serving bowl along with celery, scallion, and basil, for Mango Slaw.
3. Preheat grill and lightly oil rack. Thread mushrooms through length of caps onto skewers; season with salt and pepper.
4. Remove fish from marinade. Season with salt and pepper. Grill mushrooms and fish, turning once, until golden brown, fish is cooked through and mushrooms are tender, about 7 minutes. Transfer fish to a serving platter. When cool enough to handle, remove mushrooms from skewers and thinly slice.
5. Add mushrooms and remaining 2 tablespoons juice to Mango Slaw; toss to combine. Salt and pepper to taste. Serve fish warm with slaw on the side. Makes 4 servings.