In a large skillet, melt the butter over medium heat. Whisk in the flour, and cook while stirring for 1-2 minutes.
Slowly add in the half and half, about 1/2 cup at a time, whisking until a roux has formed and it's completely smooth. Whisk in the seasonings, and let the mixture simmer for another 2 minutes- stirring occasionally, until the sauce has thickened some.
Remove the skillet from heat, and whisk in the cheese, stirring until completely melted and incorporated.
Add the green beans, bacon, and croissants, and use a spatula to gently fold them in until incorporated.
Lightly spray a 9x13 dish with non stick cooking spray. Transfer the casserole mixture into the dish. Use a spatula to spread it out evenly. Evenly top with the fried onions.
Bake at 350 degrees for 30-35 minutes, or until golden brown and bubbly. Tent with foil and let the mixture rest for 5-10 minutes before serving, so that it 'sets'. It will still be warm and plenty creamy!