Add the butter to a large skillet over medium heat and melt.
Once butter has melted, add the mushrooms and cook, stirring often, about 5 minutes.
Sprinkle the flour over the mushrooms and stir to coat.
Stir in the chicken broth, milk, and salt, cooking over medium heat and stirring constantly, until the mixture has thickened, about 3 minutes.
Add the frozen green beans to a 5-6 quart slow cooker and top with the mushroom mixture.
Add the grated cheddar and half of the fried onions. Stir well to combine.
Cover and cook on low for 6 hours or on high for 3 hours, until cheese is melted and mixture is hot and bubbly.
Taste and add salt and pepper as desired.
Sprinkle with remaining fried onions before serving.