Prepare the macaroni cheese: Cook the pasta 2 minutes less than on the pack, then drain and put to one side.
In a small saucepan, add flour and butter. Cook for a few minutes until golden brown. Pour in the milk in stages, starting with a small amount, whisking vigorously each time. Once all of the milk has been incorporated, cook for 5 minutes, stirring constantly. Add in the grated cheese and stir until it’s melted and bubbly. Add cooked pasta and stir it all together.
Make the toasties: Heat your toastie machine. Butter each slice of bread on one side. Put a slice of bread, butter side down, on the hot plate and add a large spoonful of macaroni cheese. Top with another slice of bread - butterside up, and close the toastie machine. Cook for 2 minutes then carefully open the machine - cook for a minute or 2 longer if needed, so the bread is well cooked and golden brown. Repeat to make as many toasties as you like - but let them rest for a few minutes before serving as the the filling will be scorching hot!