Place onion and garlic in the bottom of a lightly greased 6-quart slow cooker.
Place chicken breasts and mushrooms on top.
Sprinkle the garlic powder, salt, pepper, and red pepper flakes on top.
Drizzle the chicken broth and melted butter over everything.
Cover and cook on LOW for 3 to 3 1/2 hours.
Open the slow cooker. In a small bowl, whisk together evaporated milk and flour until smooth. Pour it into the slow cooker and mix it in.
Cover slow cooker and cook on HIGH for 30 minutes to thicken the sauce.
Remove the chicken and place on a serving platter.
Add the parmesan cheese to the slow cooker and whisk into the sauce. Add the spinach and stir to mix it in.
To serve, spoon sauce over chicken.