Heat the oil in a large skillet over medium heat. Add the peppers, leeks, onions, and garlic. Season with salt and pepper to taste, and cook until the onions begin to soften, about 20 minutes, stirring occasionally. Stir in the tomatoes, cover and cook until all the vegetables are thoroughly cooked into a tender blend, almost a marmalade, about 30 minutes. Season with salt and piment d’Espelette to taste. The mixture should have a slight bite without being overly spicy.