Preheat oven to 400°. Bring 3 quarts of water to boil and add 1 tbsp salt. Peel the potatoes, slice them ¼ inch thick, add them to the water and boil for 5 minutes. Scoop them out & set aside to drain in a colander.
Drop the tomatoes in the boiling water, wait for 10 seconds, then remove & plunge them into cold water. Remove the cores, peel and halve the tomatoes crosswise. Gently squeeze out the seeds & juice. Roughly chop half of one tomato and set aside, and cut the rest into slices about ¼ inch thick.
Heat 2 tbsp olive oil in a large skillet and add the onions with the dried thyme, fennel seeds and some black pepper. Sautee over medium-high heat, stirring frequently until onions brown slightly and soften, about 3 minutes. Season with salt & set aside.
Cover the bottom of an 8 cup baking dish with 2 tbsp olive oil, then add half the onions & garlic, the chopped tomato, the olives & half the thyme leaves. Layer the potatoes & tomatoes over the onions, tucking in the remaining garlic slivers & capers, and seasoning with salt & pepper as you go. Top with the remaining onions & spoon 2 tbsp olive oil over the top. Bake, covered with foil, for 20 minutes; then bake, uncovered, for 15 minutes. Scatter the remaining thyme leaves over the top and serve.
Serves 4-6