Bring 1 cup water and 1/4 teaspoon salt to a boil in a medium saucepan; gradually stir in couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork.
While the couscous stands, heat a large nonstick skillet coated with cooking spray over medium-high heat. Combine the sesame seeds, flour, 1/4 teaspoon of salt, and black pepper. Dredge both sides of tuna steaks in the sesame seed mixture. Add the fish to pan, and cook 4 minutes on each side or until desired degree of doneness. Remove the fish from pan, and keep warm.
While the fish cooks, combine the orange juice and the next 5 ingredients (orange juice through garlic) in a small saucepan, and bring to a boil over medium-high heat. Combine 1 tablespoon water and cornstarch, stirring with a whisk until smooth. Add the cornstarch mixture to the pan, and cook for 2 minutes or until the sauce is thickened, stirring frequently. Serve the fish with the couscous and sauce.