Step 1Heat the oven to 300 degrees. Cut each tomato in half and arrange them in a single layer cut side up in a 5-quart earthenware gratin dish. Sprinkle lightly with salt and scatter the whole garlic cloves over the top. Cover generously with three-fourths cup to 1 cup olive oil (depending on the size of the tomatoes); the tops of the tomatoes should just break the surface.
Step 2Bake at 300 degrees until the tomatoes are fragrant, have browned on the surface and are beginning to shrivel, about 1 hour. Set aside.
Step 3While the tomatoes are cooking, clean the sardines. Begin by laying the fish on a board and making a small cut on the dorsal side right behind the head and straight down through the backbone. Make another incision on the belly side just behind the front fins. Holding the fish under running water, gently twist the head from the body. If you do this right, most of the innards will come away with the head. Discard these. Using the same small knife, cut a slit the length of the belly and rinse out the inside. With your thumb and forefinger, grasp the backbone near the head and pull up, using the fingers of your other hand to hold the meat in place. The backbone and larger ribs should lift cleanly away, leaving you a neatly butterflied fish with the tail attached.
Step 4In a large pie plate or baking dish, whisk together the remaining one-half cup olive oil, the minced garlic, oregano, red pepper and 2 tablespoons lemon juice. Dip the sardines in the mixture to season and then arrange them in a single layer in the mixture to marinate. Leave at room temperature for up to 1 hour, turning them over once.
Step 5Slice the cucumbers about one-eighth inch thick and season very lightly with salt and the remaining 2 tablespoons of lemon juice.
Step 6Heat the broiler. Line a broiler pan with aluminum foil and heat it for 10 minutes. Arrange the sardines on the pan skin-side up and place as close to the broiler as you can get it. Cook just until the skin begins to brown, about 3 minutes. The underside will be cooked by the heat of the pan but should still be moist.
Step 7Arrange the cucumber slices in a low mound on each of 6 plates. Place 2 grilled sardines on top and then spoon over some of the confited tomatoes, including a little of their oil. Season with a sprinkle of salt and just a drop or two of red wine vinegar onto the sardines. Serve immediately.