Heat 2 teaspoons oil over medium heat in a large pot or Dutch oven.
Add 1 chopped onion, 1 teaspoon minced garlic and ¾ teaspoon cumin seeds; season with ½ teaspoon kosher salt and cook until softened, 6–8 minutes.
Transfer onion mixture to a blender with 2 cans diced tomatoes (and their juices) and blend until smooth, 1–2 minutes.
Meanwhile, heat remaining remaining 2 teaspoons oil in now-empty pot over medium. Add 1 bag fideos and cook, stirring frequently, until toasted and fragrant, about 4 minutes. Reduce heat to low.
Carefully add blended sauce (it will splatter) and cook 2 minutes. Stir in 4 cups broth and 2 cups water; bring to a boil over high heat then reduce to a simmer over medium and cook until pasta is al dente and soup has slightly reduced, 5 minutes.
Season with salt (I add 1–1½ teaspoons salt) and black pepper, to taste. Serve with crumbled Cotija, fresh cilantro and cracked black pepper.