Place all of the dry ingredients into a food processor and blend until the seeds are roughly crushed. With the motor still running, gradually add the vinegar and blend until well mixed.
Add the stout and blend briefly.
Transfer the mustard to a bowl and leave to stand for one hour to thicken, then transfer to sterilised dry jars.
Cover the surface of each with a disc of waxed paper, waxed side down, and leave until cold. Top the cold jars with airtight lids, label and store in a cool dark place. The mustard should keep for 3-6 months.
Preheat the oven to 200C/400F/Gas 6.
For the onions, heat a separate pan until hot and add the butter and oil, then add the onions and thyme. Reduce the heat and cook slowly for 12-15 minutes, until caramelised.
Meanwhile, for the sandwich, heat an ovenproof frying pan until hot and add half the butter and oil.
Season the steaks with freshly ground black pepper. Add to the hot pan and cook for 2-4 minutes on one side. Turn the steak over and transfer to the oven for 3-4 minutes, or until cooked to your liking.
Remove the steaks from the pan, season with salt and set aside to rest for a few minutes. Reduce the oven temperature to 180C/355F/Gas 4.
Place the ciabatta into the oven for five minutes, just to warm through.
Dress the Little Gem leaves with a squeeze of lemon and a little olive oil.
To serve, cut the ciabatta loaf in half then split it lengthwise and place onto a board.
Cut the fillet steak on an angle into 1cm/½in thick slices.
Spread the mustard onto the bottom of the bread, and top with the dressed lettuce.