Cook spaghetti according to package directions. Drain.
Meanwhile, in a large skillet melt butter over medium heat. Add onions and cook until tender, about 3-4 minutes.
Whisk in chicken broth and mushroom soup until fully combined. Gradually add the evaporated milk stirring constantly until smooth. Taste and season with salt and pepper as desired.
In a 9x13-inch baking dish, mix cooked spaghetti, mushrooms, and chicken (or turkey). Pour in sauce. Top with cheese.
Bake for 20-25 minutes until cheese is melted and sauce is bubbly.
Video
Notes
Leftovers will keep in the refrigerator for 4 days and in the freezer for 2 months.