London smoked eel sandwich

For ➊
1 slice of sourdough bread
butter
30-40 g fillet of smoked eel*
1 heaped ts of horseradish cream
Dijon mustard
¼ small red onion, peeled & thinly sliced
2 tbs white wine vinegar
a good pinch of sugar

Dissolve the sugar in the vinegar. Steep the sliced onion in the pickle and let stand for 1 h.
Warm a grilling pan over a gentle heat. Lay the sour dough on the grill and brown nicely.
Cut the eel fillet into 3 largish pieces.
Butter the toasted side of the sour dough. Cut this in half and spread with Dijon mustard.
Lay on the eel and then the horseradish cream. Lay on the other piece of toast and return to the grill.
Allow the sandwich to brown, then flip and cook similarly on the other side.
Put on a plate and heap the drained pickle alongside.

Serve immediately.

*Replace with smoked trout or mackerel fillet. Or even smoked halibut.