Place chicken breasts in a large stockpot and fill it about ¾ of the way with water. Add bouillon.

Boil until chicken is cooked through (165°F), about 15–20 minutes. Remove chicken, shred or dice.

In the same pot of broth, crack the spaghetti noodles in half and cook until al dente. Drain, reserving at least 2 cups of broth (you’ll likely need around 1½ cups, but keep extra just in case).

In a skillet or Dutch oven, melt butter and sauté onion with all the seasonings (Tony’s, garlic powder, onion powder, salt & pepper) until onion is translucent, about 5–7 minutes. Add garlic, cook 1 more minute.

Pour in the heavy cream, then stir in the cream of chicken soup. Add Velveeta and stir until melted. Add about ¾ cup of the reserved chicken broth and mix well.

Once everything is smooth, stir in sour cream, the thawed and squeezed spinach, and about half of the shredded Colby Jack cheese. Add more broth as needed until the sauce reaches a creamy, pourable consistency (about 1½ cups total usually works).

Spray a 9x13 baking dish with nonstick spray.

Add cooked spaghetti noodles first, spreading evenly. Top with shredded chicken. Pour the sauce over everything and gently toss in the baking dish until noodles are fully coated.

Sprinkle with the remaining Colby Jack and mozzarella cheese. Crush the French fried onions lightly in your hands and sprinkle over the top (about 3 oz).

Cover with foil, bake at 350°F for 20 minutes.

Remove foil and bake another 20–25 minutes until bubbly and golden.

Let casserole sit 10 minutes before serving to allow sauce to thicken slightly.