Slow-Roasted Salmon with Green Sauce

Place onion, garlic, parsley, watercress, spinach, buttermilk, sour cream, lemon juice, 1 teaspoon salt, and 1/4 teaspoon pepper in the bowl of a food processor. Puree until smooth, and set aside in the refrigerator.

Preheat oven to 300 degrees. Rub the bottom of a 9-by-13-inch glass baking dish with 1/2 teaspoon oil. Set salmon fillets in the dish, and rub tops with remaining teaspoon oil. Sprinkle with remaining teaspoon salt and 1/4 teaspoon 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.