Step 1

Heat oven to 400 F.

Step 2

Wash potatoes, then slice them in half lengthwise. If any are a lot larger than the rest, slice them in thirds so that the pieces are more or less the same size.

Step 3

Bring 6 cups of water to a boil (add salt if desired). Add potatoes. When the boil returns, cook for 4 minutes, then scoop potatoes out and set them aside in a strainer.

Step 4

Drop tomatoes into same pot for about 10 seconds, then remove them to a bowl of cold water and peel off skins. Halve them crosswise and gently squeeze out seeds, then cut the walls into large pieces. Finely chop cores.

Step 5

Warm 1 Tbsp olive oil in a skillet set over medium-high. When it’s hot, add onions along with dried thyme and fennel seeds. Cook, tossing them in pan, until they begin to soften in color, after about 5 minutes. Turn off heat, add lemon zest, and season with salt and pepper.

Step 6

Coat an 8-cup gratin dish with cooking spray. Spread half the onions, the minced tomato cores, and half of the larger pieces of tomato, garlic, olives, capers, and thyme sprigs in the dish. Cover with the potatoes and the remaining tomatoes and tuck in the remaining slivers of garlic. Cover potatoes with remaining onions and wiggle in the rest of the thyme, olive, and capers. Drizzle remaining 1 Tbsp oil over potatoes, cover with foil, and bake for 25 minutes. Remove foil and bake another 10 minutes, or until potatoes are done. Put out lemon quarters for those who wish a little more acid.