1. Cut carrot lengthwise into ribbons using a vegetable peeler. Curl ribbons around fingers, and place carrot curls in a bowl of ice water.
2. Combine vinegar, juice, 3/4 teaspoon salt, chili garlic sauce, and mustard. Add 1 1/2 teaspoons sesame oil and 1 1/2 teaspoons canola oil, stirring with a whisk.
3. Heat 1 teaspoon sesame oil and 1 teaspoon canola oil in a large nonstick skillet over medium-high heat. Sprinkle fish evenly with remaining 1/4 teaspoon salt and pepper. Add fish to pan; cook 5 minutes on each side or until desired degree of doneness. Remove fish from pan.
4. Heat remaining 1/2 teaspoon sesame oil and remaining 1/2 teaspoon canola oil in pan over medium-high heat. Add onions to pan; sauté 5 minutes or until lightly browned, turning occasionally.
5. Combine lettuce, bok choy, and tomatoes in a large bowl. Drain carrot curls; add carrot curls to lettuce mixture. Drizzle vinegar mixture over lettuce mixture; toss gently to coat. Arrange 1 1/2 cups lettuce mixture onto each of 4 plates. Cut each tuna steak into thin slices. Arrange 1 tuna steak and 4 onions on each serving; sprinkle each serving with 3/4 teaspoon sesame seeds.
Wine note: What to pour with these diverse flavors? Dry rosé's bold fruitiness stands up to the richness of the sesame oil, the impact of chili sauce, and the density of tuna. I love Côtes-du-Rhône rosés from France. In particular, the 2006 Domaine de la Janasse Côtes du Rhône Rosé is fantastic with this dish ($14). —Karen MacNeil