Season the salmon with the salt and rub over the lemon rind. Leave to marinate for 10 minutes.
Take a roasting tin large enough to fit the salmon fillets snugly in one layer and cover the bottom with olive oil. Place the salmon fillets, skin-side down, in the tin. Pour over the remaining olive oil, which should just cover the fish (if it doesn’t quite cover it, you will need to baste a couple of times during cooking). Tuck the garlic cloves and thyme sprigs around the fish and cover with foil.
Place in a preheated oven, 140°C (275°F), Gas Mark 1, for 15 minutes until the fish is just cooked through.
Whisk together the vinegar and extra virgin olive oil, then season well. Toss with the watercress. Lift the salmon out of the oil and drain on kitchen paper. Serve 2 roasted garlic cloves per person with the watercress salad.