Step 1: Make the rub: Heat a dry cast iron skillet over a medium flame. Add the peppercorns, mustard seeds, fennel seeds, and grains of paradise (or whole allspice berries, if using). Cook until fragrant and lightly toasted, 1 to 3 minutes, stirring with a wooden spoon. Do not let the spices burn. Transfer the spices to a heatproof bowl to cool.
Step 2: Place the spices in spice grinder and coarsely grind, running the machine in short bursts. Work in the salt and hot pepper flakes. This will give you about 2/3 cup rub—a little more than you need for the tuna, but it’s easier to roast the spices in larger batches.
Step 3: Trim any skin or dark or bloody spots off the tuna. Rinse the tuna under cold running water, then blot it dry with paper towels. Place the tuna in a baking dish. Lightly rush on all sides with sesame oil. Sprinkle the rub all over the tuna on both sides, patting it onto the fish with your fingertips.
Step 4: Set up the grill for direct grilling and preheat to high.
Step 5: When ready to cook, brush and oil the grill grate. Arrange the tuna on the hot grate and grill until the outside is nicely seared, about 1 minute per side (4 minutes in all), turning the loins with tongs. If using steaks, grill for about 2 minutes per side, rotating each steak a quarter turn halfway through grilling on each side to create an attractive crosshatch of grill marks.
Step 6: Transfer the grilled tuna to a cutting board and let it rest for 1 minute. Carve crosswise into 1/4 inch thick slices. Fan the slices out on a plate. Squirt zigzags of Wasabi Cream Sauce over the tuna, sprinkle with the scallions and sesame seeds, and serve at once. Serve any remaining Wasabi Cream Sauce on the side.