In a food processor, combine cooked chickpeas, walnuts, and taco spices. Pulse until crumbly for the taco crumbles.
Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente. Drain and set aside.
In a skillet, heat olive oil over medium heat. Sauté chopped onion for 3-4 minutes. Add minced garlic and cook for another minute.
To the skillet, add taco crumbles, diced tomatoes, tomato paste, tomato sauce, dairy-free milk, nutritional yeast, and tamari. Stir and simmer for 10 minutes.
Add cooked spaghetti to the skillet. Toss to coat noodles with sauce. Add more dairy-free milk if sauce is too thick.
Serve hot, garnished with cilantro, olives, red pepper flakes, vegan cheese, jalapeños, or sour cream.