ROMESCO SAUCE: Heat oven to 450ºF. Rub tomatoes and peppers with oil, and place on a parchment-lined baking sheet. Roast peppers until skins are blistered and the flesh is soft, about 20 minutes. The tomatoes will take about 30 minutes. When cool enough to handle, peel peppers, slip skin off tomatoes, and core.
Heat 1 tablespoon of olive oil in a small skillet; add bread crumbs and garlic and toast over medium heat until golden, 3 to 4 minutes. Combine bread crumbs, garlic, peppers and tomatoes in a food processor, and puree until smooth. Adjust the seasoning with salt, cayenne, sherry vinegar, and lemon juice to taste.
POTATO CAKES: Place potatoes in a pot of cold water, and bring to a boil. Lower the heat, and simmer, uncovered, until they’re tender but not quite cooked through, about 25 minutes. Drain, and set aside to cool. Peel potatoes, and grate by hand or in a food processor.
Heat oil in a medium-size skillet, and add leeks, ¼ teaspoon salt, and a few pinches of pepper. Cook over medium heat until leeks begin to soften, about 3 minutes.
Combine leeks, potatoes, ½ teaspoon salt, and a pinch of pepper in a bowl. Beat eggs and crème fraîche together, and mix into potatoes along with cheese and flour.
Form into little cakes 3 to 4 inches in diameter and ½ inch thick. Cook in a well-oiled skillet or on a griddle over medium heat, allowing cakes to crisp and turn golden, about 5 minutes per side. Serve immediately, with romesco sauce.