Place onion, garlic, parsley, watercress, spinach, buttermilk, sour cream, lemon juice, 1 tbs salt, and 1/4 tbs pepper in the bowl of a food processor; puree until smooth, and set aside in the refrigerator.
Heat oven to 300 degrees.
Rub the bottom of a 9 x 13-inch glass baking dish with 1/2 tbs oil.
Set salmon fillets in the dish, and rub tops with remaining tbs oil.
Sprinkle with remaining tbs salt and 1/4 tbs of pepper.
Pour 1/4-cup water in dish to cover bottom in a thin layer.
Cook fish until just cooked through, 20 to 25 minutes.
Serve immediately over bed of watercress drizzled with cold herb sauce.
Extra sauce can be refrigerated up to 4 days.
Ready in 40 min.