Grind mustard seeds to a fine powder. Add 1 tablespoon of warm water to make a thin paste. Let stand for 30 minutes. Rinse tuna and pat dry. Place tuna in a shallow dish. In a food processor, blend together garlic, ginger, and 1 tablespoon of water. Add mustard paste, cayenne and lemon juice. Process to combine. Add mustard oil and yogurt. Process to combine. Rub mixture over fish and marinate for 1 hour. Heat skillet over moderate heat, add tuna and cook until tuna is is browned on the outside about 5 minutes per side. Baste while cooking. Garnish with lemon wedges and serve.