In a blender, combine the onion, tomatoes, garlic, pepper, and veggie broth and purée until smooth.
In a Dutch oven over medium heat, warm the oil. Add the alphabet pasta and heat, stirring, until lightly toasted, 1–2 minutes.
Add the puréed mixture, stir well, and cook, stirring, until the mixture thickens and darkens, about 5 minutes.
Add the water, stir well, and bring to a boil, stirring occasionally. Reduce the heat to low, cover, and cook until the pasta is cooked, about 10 minutes.