Place the diced tomatoes with green chiles in a small food processor or blender and pulse until coarsely blended; set aside.
Heat the oil in a large Dutch oven or stockpot over medium-high heat.
Add the ground meat and minced onion and cook, breaking the meat into bite-sized pieces, until browned and cooked through, about 6–8 minutes; drain any excess fat if needed.
Return the pot to the heat and stir in the taco seasoning, blended tomatoes, tomato sauce, and 2 1/2 cups water; bring to a boil.
Once boiling, add the broken spaghetti, reduce heat to medium, and simmer uncovered, stirring often, until the noodles are al dente, about 15–18 minutes; add additional water 1/4 cup at a time if liquid is evaporating too quickly.
When the noodles are tender, remove the pot from heat and stir in the grated cheddar; let sit a few minutes to melt.
Serve immediately topped with chopped cilantro and any desired garnishes such as sour cream, avocado, tomato, green onion, jalapeño, or black olives.
