Preheat the oven to 350 degrees.
Grease 9 by 13 casserole dish.
In a large pot, bring 12–16 cups of water to a boil with 3 bouillon cubes. Add the spaghetti and cook until al dente. Before draining, ladle out 1–2 cups of the pasta water and set aside. Drain pasta and return it to the pot . Pro Tip: This step is key! Cooking in bouillon makes the noodles taste like they were simmered in chicken broth, and the reserved water will keep your casserole creamy.
Return the pasta to the pot.
Add the chicken, 1 cup of the cheese, Ro-tel (and liquid), salt, cream of chicken soup, dried basil, and diced onion.
Stir gently.
Add 1 cup of the liquid and, if the pasta still seems a bit dry…the remaining ½ cup.
Dump the pasta mixture into the greased casserole pan.
Top with the remaining cheese.
Bake for 45min - 1 hour (if the cheese starts getting to brown, cover with aluminum foil in the last 15 minutes) or until hot and bubbling.
Let cool for 15 minutes before serving.