Combine the milk, onion, bay leaf, cloves and nutmeg in a heavy-gauge, non-corrosive saucepan over medium heat. In a separate pan create a roux by cooking the butter and flour for a few minutes, stirring continuously, without browning, until it emits a nutty aroma. Remove from the heat. When the milk is fairly hot, pour a small amount of it into the cooled down roux, stirring until the milk is thoroughly blended. Return this to the remaining milk, and simmer for 15 minutes, stirring frequently. Remove from heat and season, to taste, with salt and white pepper. Whisk in mustard and cover with a piece of plastic wrap to prevent a skin from forming.
Heat the grill to medium heat. Prepare the tortillas by laying 1 or 2 slices of cheese on top of each and set aside briefly. Slowly cook the bratwurst over medium heat, turning as needed to prevent them from getting charred. (They tend to split when the heat is too high.) When they are browned and cooked through, which takes 10 to 15 minutes, transfer them to the cheese lined tortillas, and roll up the bratwursts so that the cheese will melt and hold everything together. Place the rolled bratwursts in a skillet over medium heat, allowing the tortillas to toast slightly. Serve with mustard sauce.
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