Blend dry mustard ingredients in a food processor until seeds are roughly crushed.
Gradually blend in vinegar until well mixed.
Add stout and blend briefly.
Let mustard stand for one hour to thicken before transferring to jars.
Preheat oven to 200°C.
Slowly cook onions in butter and oil with thyme for 15 minutes.
Heat half the butter and oil in an ovenproof frying pan.
Season steaks with pepper and sear in the pan for 4 minutes.
Transfer pan to oven and cook steaks for 4 minutes.
Season steaks with salt and rest; reduce oven to 180°C.
Warm ciabatta in the oven for five minutes.
Dress lettuce leaves with lemon juice and olive oil.
Slice steak on an angle into 1cm thick slices.
Assemble sandwiches by spreading mustard on bread and layering with lettuce, tomato, steak, and onions.