Heat the oil in a small saucepan,
add the onion, cover and cook, stirring occasionally, for 10
minutes.
Add the sugar and vinegar and
cook, uncovered, for a further 7 minutes, or until the onion is soft
and lightly browned.
Remove from the heat and stir in
the thyme.
Meanwhile, combine the mustard and
mayonnaise in a small bowl.
Lightly butter the bread.
Place the rocket leaves in a bowl,
drizzle with the olive oil and season with salt and cracked black
pepper.
Heat 1 tbsp of the extra oil in a
frying pan over high heat, then cook the steaks in batches for 2
minutes each side, adding the remaining oil if necessary.
Season and keep warm.
Place the rocket leaves on four
slices of the buttered bread.
Top with steak, slices of tomato,
the onion and a dollop of the mustard mayonnaise.