Preheat your oven to 210 degrees celsius (fan-forced) and line a tray with baking paper.
Place the flour, butter and salt into the Thermomix bowl and mix for 10 seconds on speed 9 until mixture is the same consistency as bread crumbs.
Add the milk and mix for 7 seconds on speed 4 to combine.
Use the kneading function to knead the dough for 2 minutes
Turn the dough out onto a floured bench and using a rolling pin, roll the dough out until you have a 30cm x 40cm rectangle.
Spread the butter and mustard (if using) over the dough, leaving a 2cm gap around the edges.
Brush one long edge with a little milk.
Scatter the ham and cheese over the dough.
Starting at the long side (without milk) carefully roll the dough until enclosed.
Cut into 12 slices – approximately 2cm thick.
Place on a baking tray and sprinkle with a little extra grated cheese.
Bake in oven for 25 minutes or until golden brown on top and cooked through.