Lay scallops on a paper towel–lined plate and allow to come to room temperature, about 15 minutes. Season with 1/2 tsp. salt and let stand 5 more minutes. "Seasoning before cooking develops the best flavor," says Clark. Blot dry.
Meanwhile, make relish: If using fresh tomatoes, dip them in a large pot of boiling water, 10 seconds. Let cool, then peel. (If using canned tomatoes, don't boil.) To seed tomatoes, simply cut them in half and squeeze. Core and dice fine.
Heat 2 tbsp. butter in a large skillet over medium heat until it begins to brown. Add onion, fennel, and a pinch of salt and cook until vegetables are soft, about 10 minutes. Add garlic and cook 2 minutes; add cumin and cook 1 minute more, stirring often. Stir in tomatoes and cook until sauce is glossy and thickened, about 8 minutes. Stir in the remaining butter "to emulsify the sauce a bit," then add parsley. Season to taste with salt and pepper.
Heat a large frying pan (cast-iron is best; with nonstick, you won't get a good crust) over medium-high heat until hot. Swirl in oil, then arrange scallops in a single layer in pan, with ample space around each. Cook without moving scallops until golden and crusty underneath, about 2 minutes; then turn and cook for about a minute. Remove from heat and let scallops sit in pan 1 minute; transfer to a plate, starting with the thinnest scallops.
While scallops are cooking, spoon some relish into gratin dishes. Add a scallop or two to each. Top each with a dab of relish and a snippet of fennel fronds. "Any garnish should be part of the dish, and it should be eaten."