Melt 3 Tbsp. butter in a large pot over medium heat. Make sure the milk is easily reachable and sprinkle the flour over the melted butter and whisk in. Whisk for a few seconds until the flour and butter have combined to a silky mass. Add about 50ml (1/4 cup) of milk and whisk continually as it heats up. Once it's reached a uniform consistency, add another 50ml of milk, whisk and repeat until you have a creamy sauce. Stir in the spices and mustard.