Place the yellow and brown mustard seeds into a bowl.
Heat vinegar and honey in a pan until the honey dissolves.
Pour the honey mixture over the seeds and mix thoroughly.
Stir in the ale, cover, and set aside for four hours or overnight.
Blend the mixture in a food processor until smooth.
Spoon the mustard into jars and seal.
Bash the steaks between sheets of film to flatten slightly.
Spread 2 tablespoons of mustard over each steak.
Melt butter with 2 tablespoons of oil in a hot pan.
Fry steaks for 3 to 4 minutes per side and rest for 5 minutes.
Sauté aubergines, shallots, and rosemary in remaining oil for 6 minutes.
Serve steaks with extra mustard and a side of aubergine mixture.