Preheat oven to 400°F. Quarter onion. In a shallow baking pan toss onion, tomatillos (in husks), chiles, and garlic cloves with 2 tablespoons olive oil until coated. Roast vegetables in middle of oven until tender, about 25 minutes.
Wearing rubber gloves, peel chile. Cut off top of chiles and remove seeds and ribs. Discard husks from tomatillos and peel garlic. In a blender purée roasted vegetables and all remaining coulis ingredients except water, adding just enough water, 1 tablespoon at a time, if necessary to facilitate blending. Season coulis with salt.
With a paring knife make 3 slits in top of each salmon fillet and stuff each with a basil leaf. With a mortar and pestle or an electric coffee/spice grinder coarsely grind peppercorns. Brush salmon with extra-virgin olive oil and sprinkle with ground peppercorns and sea salt. In a large nonstick skillet heat oil over high heat until hot but not smoking and sear salmon fillets, turning occasionally, until browned on all sides and just cooked through, about 7 minutes total. Serve salmon with coulis.