1. Preheat oven to 350 degrees F. Put onions, water, salsa, and diced tomatoes in medium pot and simmer until onions look transparent, about 5-6 minutes.
2. Remove from heat and quickly add tofu and couscous to pot. Stir together, put lid on, and let sit for 3-5 minutes.
3. Meanwhile, toast taco shells in oven. Fill taco shells with couscous/tofu mixture and top with more salsa.