In a blender, combine the onion, tomatoes, garlic, pepper, and vegetable broth and purée until smooth.
In a Dutch oven over medium heat, warm the olive oil. Add the alphabet pasta and heat, stirring, until lightly toasted, about 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.
Season with salt, ladle into bowls, and serve.