Cook the Spaghetti: Fill a large pot with salted water and bring it to a boil. Add the spaghetti and cook it until just al dente—usually 1–2 minutes less than the package says. Drain it well and set it aside to cool slightly. You want it warm but not steaming hot when mixing with the cheese.
Prepare the Chicken and Veggies: While the pasta is boiling, chop your cooked chicken into bite-sized pieces. Dice the bell pepper and onion into small, even pieces so they soften properly while baking. If you prefer a more mellow flavor, you can sauté the onions in a little butter until translucent.
Mix Everything Together: In a large mixing bowl, combine the drained Rotel tomatoes (with their juice), cream of mushroom soup, Velveeta cubes, chopped onion, bell pepper, and chicken. Stir in the cooked spaghetti gently. Make sure the cheese is distributed throughout the mixture. Season with salt and pepper to your taste.
Transfer to the Baking Dish: Pour the entire mixture into a greased 9x13-inch casserole dish. Use a spatula to press it down gently and even out the surface. Don’t worry if it looks a little chunky—once it bakes, the cheese will melt into a smooth, creamy sauce.
Bake Until Bubbly: Preheat your oven to 350°F (175°C). Bake the casserole uncovered for about 30 to 35 minutes, or until it’s hot and bubbling around the edges. The Velveeta should be fully melted, and the top may begin to lightly brown. If you like it crispier on top, broil for 2–3 minutes at the end, keeping a close eye.
Cool Slightly and Serve: Let the casserole sit for about 5 minutes before serving. This helps the sauce thicken and makes it easier to slice or scoop. Garnish with a little chopped parsley or even shredded cheddar if you want a pop of color on top.