In a medium soup pot, heat the olive oil, then add the potatoes. Season with salt and pepper, and then the Piment d'Espelette. Lightly brown, 5 minutes, while you chop the remaining vegetables. Add the garlic, zucchini, fennel and onion to the soup as they are chopped. Add the tomatoes and cover the pan to soften the vegetables and burst the tomatoes, 5 to 8 minutes. Add the stock and passata, and mix to combine. Cool and store for a make-ahead meal.
Reheat over medium heat and garnish with the reserved fennel fronds and a drizzle of olive oil. Serve with warm bread.