For this recipe I make a levain in the morning and let it activate all day. No need to fuss. Just make and get on with your life.
I then make the dough in the evening using the active levain.
Place all dough ingredients into the Thermomix bowl and knead 3 min/Interval/dough setting.
Tip out dough into a large oiled bowl, cover and leave overnight or 12 hours. You will have a taut firm dough but it will clearly have risen.
When you are ready to bake, you need to do the pull and fold technique as described on video and then divide the dough into two equal pieces. Cover with a floured tea towel and allow to proof while you prepare the rest of the recipe.
Preheat either a pizza stone or cast iron flat dish in an oven set as high as you can get it. I have the option of 275°C. Anything over 250°C is good. The hotter your oven during pre-heating the better.
Place the garlic into the Thermomix bowl and chop 3 sec/speed 8. Add the spinach and chop 3 sec/speed 5.
Prepare the steak by drying it with a paper towel and seasoning with salt and pepper.
Roll the first piece of dough into a large rectangle, not thinner than 2cm thick, especially at the 'bottom' of the loaf, where the meat will sit. You can make the edges slightly thinner as the they will be doubled and create a thicker top when cooked anyway.
Spread the entire piece of dough with mustard. (If you prefer you could use tomato sauce.)
Place the chopped garlic and spinach mixture into the centre of the dough. Place the steak on top. Top with sliced cheese, sliced mushroom and grated parmesan. All of this should make a tidy parcel, so don't go crazy. You also don't want to tear any of the dough and have a leakage.
Fold the dough over to create a parcel. Seal all the joins very well. Sprinkle with cornmeal, salt flakes and pepper.
Transfer to the oven using a pizza peel or similar and bake on the preheated stone or cast iron for 25 minutes for medium rare, longer for well done. I wouldn't recommend a rare steak sandwich as it would be a bit difficult to eat.
Let it cool for at least 10 minutes (you are resting the meat) before slicing and devouring.