Preheat the broiler. Brush tuna steaks on both sides with olive oil. Season with salt and pepper. Broil the tuna steaks for 2 minutes per side, until just rare. Let tuna cool to room temperature, then finely chop the tuna with a knife.
While tuna cools, saute red onion until soft. In a large bowl, combine the tuna with red onion, olives, capers, and dill. Fold in mayonnaise, mustard, and lemon juice and pepper. Form tuna mixture into cakes, and chill for 30 minutes or more.
Combine the sesame seeds and flour. Dust the tuna cakes with the flour/seed mixture. Heat 1/4 inch of olive oil in a large skillet until shimmering. Add 3 or 4 tuna cakes, and saute on each side for three to four minutes, until golden brown. Drain on paper towels, and serve.