First of all, the sauce: mix together in a bowl the mayonnaise, together with capers, ground parsley and, if you want, a pinch of salt. Then, chop the red onion from Cavasso into small pieces.
After that, cut the buns into halves and toast the bread in a non-stick pan. Once you removed the remaining crumbs, you can add some extra virgin olive oil and use the same pan to cook the trout medallions, until they brown a bit. If you want, you can add a bit of salt.
The last step consists in creating the hamburger. Spread the sauce on the bread, then, on each half of the bun, put a trout medallion, a slice of Montasio DOP, some lettuce, and the red onion. After closing the buns with the remaining halves, you are ready to taste your hamburger.
After the first bite, however, you can drink a glass of Friulano Grave DOC, to accompany this greedy recipe.