Cut the loin in 4 equal, thick 'steaks'. Trim off any sinew. On a plate. Mix together all the spices. Season the tuna steaks with salt then dredge them in the spice mix. In a very hot saute pan coated lightly with oil, sear the tuna until it becomes dark brown then flip, about 3 minutes a side. The tuna will still be rare and slightly warmed. Let rest 3 minutes before slicing in half on the bias.
For the Black Bean-Shiraz Butter Sauce: In a non-reactive saucepan, add the shallots, black beans, ginger and wine. Bring to a simmer and completely reduce the wine. When wine is gone, add the cream and bring to a simmer. Immediately transfer to a blender and blend at high speed. Add the butter, 1 piece at a time. Season with salt and pepper and check for seasoning. Keep warm in a hot water bath. Plating: On large, round plates, place a mound of potatoes in the center. Top with the tuna and top that with the salad. Place the sauce around the tuna and drizzle with a little vinaigrette.