Lightly toast pine nuts in a small nonstick pan over medium high heat, about 3-5 minutes; set aside.
Heat 2 teaspoons of oil over medium heat in a large pan and sauté onion until it start to soften, about 4 minutes. Add in the garlic and continue to cook another minute. Stir in both yellow and red peppers, water, and 1/4 teaspoon of salt.
Lower heat and gently simmer until peppers are tender crisp and water is absorbed, about 10-12 minutes. Remove pan from heat and stir in the parsley, toasted pine nuts, vinegar, and 1/8 teaspoon of pepper; place in a bowl and keep warm or cool to room temperature.
Place grill rack 4-6 inches from heat and preheat grill to medium high heat.
Using a pastry brush, coat tuna steaks with remaining teaspoon of oil and sprinkle with remaining 1/4 teaspoon of salt and 1/8 teaspoon of pepper. Place on grill and cook until opaque throughout with a small amount of pink showing in center, about 3-4 minutes per side, turn only once. Serve topped with onion pepper salsa.