In a blender, working in batches if necessary, whirl tomatoes with 1/2 teaspoon salt until smooth. Pour into a strainer lined with two layers of cheesecloth (four if it's loosely woven) and set over a large bowl. Enclose nested containers in plastic wrap and chill, without stirring, at least 12 hours and up to 2 days to drain. Gather cloth edges and twist gently to squeeze out any remaining clear liquid, stopping when any red is released. Pour clear tomato water into a glass measure; you need 3 to 3 1/2 cups. Save red purée for other uses, such as pasta sauce.
In a 3- to 4-quart pan over high heat, bring about 2 quarts water to a boil. Add pearl onions and cook just until barely tender when pierced, about 3 minutes. Lift out with a strainer or slotted spoon and rinse under cold running water until cool enough to handle. Trim root end from each onion and squeeze out of peel. Add peas to boiling water and cook until barely tender to bite, 2 to 3 minutes (see notes); drain.
Rinse salmon and pat dry. Sprinkle all over with salt and pepper. Lay pieces on a well-oiled grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Cook, turning once, until barely opaque but still moist-looking in center of thickest part (cut to test), 6 to 8 minutes total.
Meanwhile, in a microwave oven on full power (100%), heat tomato water until steaming, 2 1/2 to 3 minutes.
Set a piece of salmon in each of four wide, shallow bowls. Pour tomato water around fish and distribute onions, peas, and cherry tomatoes in water.