Heat 2 teaspoons oil over medium heat in a large pot or Dutch oven.
Add onions, garlic and cumin seeds; season with ½ teaspoon salt and cook until softened, 6–8 minutes.
Transfer onions mixture to a blender with canned tomatoes and juices and blend until smooth, 1–2 minutes.
Meanwhile, heat remaining remaining 2 teaspoons oil in now-empty pot over medium. Add fideo 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 broth and 2 cups water. Bring to a boil over high heat then reduce to a simmer over medium; cook until pasta is al dente and soup has slightly reduced, about 5 minutes. Stir in chicken and peas and cook until warmed through, about 2 minutes.
Season soup with salt (I add 1–1½ teaspoons salt) and black pepper to taste. Serve with crumbled Cotija, fresh cilantro, and cracked black pepper.