Marinate salmon in 3 cups ponzu for 30 minutes to 1 hour, turning occasionally.
For each serving, to order, pat 1 salmon filet dry; brush with lightly with vegetable oil and season with salt and pepper. Toss 2 lemon slices and 2 scallions with vegetable oil, season with salt and pepper. On a gas or charcoal grill, grill salmon on flesh side, turning once to form crosshatch grill marks. Turn and grill on skin side until done. Grill lemon slices and scallions until grill-marked; cut scallions into long slivers. Plate salmon; garnish with lemon slices and scallions. Drizzle with 2 tablespoons ponzu and a few drops of sesame oil; sprinkle with shichimi togarashi.