Pre-heat the oven to 425. Bring a large nonstick skillet over medium heat and add the sausage, fennel and red pepper flakes. Cook the sausage, breaking it up with a cooking spoon or spatula, until browned and broken into small pieces. Set aside. In a large mixing bowl (or a blender – you can blend this mixture until well combined), combine the milk, flour, eggs and salt and whisk together until smooth and well combined. Place the butter into the bottom of a 9×13 baking dish and place in the pre-heated oven for 5-10 minutes until the butter is completely melted. Remove the dish and tilt so that the butter spreads across the bottom of the dish. Pour the flour/egg mixture across the bottom of the dish and then sprinkle the cooked sausage and shredded cheese over the top. Place back in the oven to bake for 20-25 minutes until the cheese starts to brown and the ingredients are cooked through.