Season the salmon with salt and cayenne pepper, drizzle with lemon juice and let sit in the refrigerator for 10 minutes.
Cut the onions, carrots and zucchini into even strips and fry them in the pan with the oil. Deglaze with the vinegar and vegetable stock. Simmer on medium heat until the liquid has completely reduced. Add the sugar beet syrup, season with salt and pepper and stir well.
Shape the aluminum foil into a baking dish with a high rim. Make the edge high enough that it will later become the lid. Spread the vegetables on 4 aluminum casserole dishes and place the salmon on top. Seal the edges over the salmon to form a packet and place it on the grill.
After approx. 15-20 minutes, remove the salmon from the grill, open the packet and place the herb butter on the salmon. If you like, you can add a lemon wedge.