Pre-heat the oven to 350° F, and grease a 9x13 baking dish with cooking spray.
In a large skillet, melt the butter and sauté the onion, bell pepper, and mushrooms until they’re tender, probably about 15 minutes.
Then add the flour and blend it in with the veggies really well.
Add in the milk and chicken broth, cooking on low ’til the sauce begins to thicken.
Finally, add in the salt, pepper, and cheeses, and cook until the cheese is melted.
Combine the cheese sauce, shredded turkey, and noodles in a large mixing bowl, and spread the mixture in the greased 9x13 baking dish.
Bake at 350° F for about 30 minutes, or until the casserole is heated through and bubbly.
Serve while warm.