The onions can be braised 2 days ahead and kept in the fridge.
Put the vinegar and sugar into a heavy-based saucepan and bring to the boil, allowing the sugar to dissolve. Add the sliced shallots to the warm liquid, stir through, then tip into a dish and cover with cling film. Set aside.
Arrange the smoked salmon in the centre of each plate, making sure that the slices slightly overlap, and set aside.
Put the frisée salad, the chives, capers, a little of the shallot vinegar and some of the shallots into a mixing bowl and toss together. Spoon the mixture over the salmon and serve with a few twists of black pepper, a drizzle of oil and lemon wedges on the side.