Remove any bones or skin from the fish fillets. Lightly oil a sheet of foil and place the fish in the centre. Season with salt and pepper, pour over the verjuice and lay the thyme on top. Make a loose-fitting parcel by folding the top edges and sides. Place on a baking tray and cook in the oven for 15 minutes. Allow to cool and drain off any liquid.
Heat a little oil in a small saucepan over low heat and cook the onions until soft but not coloured.
In a mixing bowl, grate the cooked potato and add the cooked onion and flaked cooked snapper. Add the chopped coriander and parsley, mix well and check for seasoning. Shape the mixture into four even rounds (use an egg ring) about 1½ cm thick, and refrigerate for about ½ an hour.
Increase oven temperature to 240°C. Arrange the four rösti on an oiled baking tray, evenly sprinkle over the grated parmesan and drizzle over a little extra virgin olive oil. Bake for 20-30 minutes or until beautifully bronzed.
Serve immediately with salsa verde and a crisp salad.
Finely chop the capers, cornichons, anchovies and herbs and put them into a bowl with the grated garlic.
Add the remaining ingredients, making sure the seasoning and vinegar is well balanced. Stir well.