Prepare a grill for indirect heat: If using a gas grill, turn all the burners on high, close the lid and heat until the internal temperature is 400 degrees. For a two-zone grill, turn off one side. For a three-zone grill, turn off the center zone. If using a charcoal grill, remove the top rack and light the charcoal; when the charcoal is covered with ash, mound on one side of the grill. Return the top rack.
Place the tea in the middle of a 12-by-12-inch piece of heavy-duty foil. Fold up into a rectangle about 4 by 5 inches. Using a small sharp knife, poke about 5 holes in the top. Rub the salt and lemon zest all over the salmon fillet. Place the onion slices on a plate. Drizzle the salmon and the onion with EVOO; season the onion with salt and pepper.
Place the foil packet with the tea under the grate directly on the flame of a gas grill or directly on the hot coals of a charcoal grill. Cover; let the tea begin to smoke, 5 to 10 minutes. Place the salmon, skin side down, on the unlit side of the grill. Cover and let grill until just opaque in the center, about 10 minutes. Lift the fillet from the grill, with or without the skin. If serving with the skin, it may need a few more minutes on the grill to crisp. Place on a platter; let rest.
Grill the onion slices on the hot side of the grill, turning occasionally, until charred in spots, about 6 minutes. Add to the platter with the fish. Place the bagels, cut side down, on the grill and cook until toasted, 1 to 2 minutes. Coarsely flake the salmon and serve with the bagels, onions, cream cheese, capers and toppings.