Heat 2 tablespoons olive oil in a large pot over high heat until shimmering.
Add the diced onion and red pepper and cook 3–5 minutes, stirring occasionally, until the onion is translucent; push the vegetables to the edges of the pot.
Add the ground beef to the center of the pot and sear about 1 minute per side; then break it apart with a wooden spoon and cook until mostly browned with only a little pink remaining.
Stir in the minced garlic, tomato paste, taco seasoning, and beef bouillon powder; season with salt and pepper (suggested 1/4 teaspoon salt and 3/4 teaspoon pepper) and cook until the beef is fully browned.
Pour in the canned tomatoes, enchilada sauce, and chicken broth and bring the mixture to a full boil.
Break the spaghetti in half and submerge it in the boiling liquid. Reduce heat as needed to maintain a gentle boil and cook 10–12 minutes, stirring often and scraping the bottom to prevent sticking.
About 8–10 minutes into cooking, reduce the heat to medium to avoid excessive evaporation so the pasta can finish cooking in the sauce.
When the spaghetti is al dente, remove the pot from heat; the sauce should be saucy but not dry. Use tongs to toss the pasta for about a minute so the sauce thickens and the pasta becomes perfectly tender.
Allow the mixture to cool slightly, then gradually stir in the shredded cheddar 1/4 cup at a time, tossing until fully incorporated and smooth.
Serve immediately.