Roast fish with lemon zest

I used fresh Atlantic haddock, but whatever white fish is in season where you live will do.

To make the lemon zest, wash and grate the zest off two lemons. Press the juice into a glass. Melt some butter in a pan, add the zest, and a bit of sugar (a table spoon or so) and let it caramelize. Then add the lemon juice and an equivalent quantity of balsamic vinegar and let the liquid boil down to about half. The color will 

be dark like the vinegar and the consistency thick. 

You can whisk in olive oil to make it more liquid if desired, or serve it quite syrupy on the side as in this picture (served here with mashed potatoes and celeriac).