Boil the spaghetti using the package’s al dente guidance. A strand should bend easily but still offer slight resistance when bitten.

Whisk the egg in a medium bowl while the spaghetti cooks, then incorporate the cooked chopped chicken, sour cream, condensed soup, Parmesan, Monterey Jack, thoroughly squeezed spinach, half the fried onions, garlic, and onion powder. The filling should look thick and evenly mixed, with no visible streaks of egg.

Drain the cooked spaghetti well, then fold it through the chicken filling until the strands are coated. Spread apart any tangled bundles so the sauce reaches the pasta throughout.

Fill the greased 9x13 baking dish with the pasta mixture and secure foil over the top. An even layer helps the center and edges heat more consistently.

Place the covered dish in the 350°F oven for 30 minutes. The sauce should be hot and beginning to bubble around the edges when you lift the foil.

Lift off the foil and distribute the reserved fried onions across the surface. A scattered, even layer gives the topping room to brown rather than steam in piles.

Finish baking uncovered for another 5 minutes, or until the onions become crisp and golden brown. Watch for deep golden edges and remove the dish before the topping turns dark.