Spray a large deep skillet with nonstick cooking spray and heat over medium.
Add the finely diced onion and minced garlic and cook until the onion is softened, about 2–3 minutes.
Add the ground turkey, season with salt and pepper, and break it up with a spoon while cooking until no longer pink.
Stir in the taco seasoning, Rotel, and chicken stock, then add the broken spaghetti pieces and bring the mixture to a boil.
Reduce heat to a simmer, cover, and cook until the spaghetti is softened and nearly al dente, about 7 minutes.
While the pasta cooks, whisk the evaporated milk and flour together until smooth.
When the spaghetti is al dente, stir in the milk-flour mixture and cook, stirring, until the sauce thickens, about 1–2 minutes.
Stir in the grated cheese, corn, and chopped green onions until the cheese melts and ingredients are heated through.
Divide into four portions and serve immediately.