Preheat the oven to 375°F (190°C) and heat a 9-inch ovenproof skillet over medium-high heat.
Add the oil to the skillet, then sauté the diced onion and minced bell pepper for 5–6 minutes until the onion is softened and translucent.
Add the ground beef and minced garlic to the skillet and cook, breaking up the meat, until no pink remains.
Stir in the taco seasoning: chili powder, oregano, ground cumin, paprika, garlic powder, onion powder, crushed red pepper flakes, plus 1/2 teaspoon salt and 1/2 teaspoon black pepper; reserve about 1/2 teaspoon of the seasoning if you prefer to add a little later to the sauce.
Add the corn and cook 1–2 minutes to warm through.
Pour in the diced fire-roasted tomatoes with their juices, stir, and cook until the tomatoes have softened and the sauce has thickened slightly, 4–6 minutes.
Taste and adjust seasoning with the remaining reserved seasoning, salt, or pepper if needed.
Add the cooked spaghetti directly to the skillet and toss gently to coat the pasta evenly with the sauce.
Sprinkle the shredded cheddar evenly over the top, then transfer the skillet to the preheated oven and bake about 20 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let rest a few minutes, then garnish with dollops of sour cream, sliced jalapeño, minced cilantro and minced green onions; add chili flakes if desired and serve immediately.