How to Make It
In a saucepan, combine the oil, thyme and garlic. Add the potatoes and heat slowly until they begin to sizzle. Cook over moderate heat until the potatoes are barely cooked, 5 minutes. Remove from the heat and let stand for 10 minutes. Using a slotted spoon, transfer the potatoes to a plate and season with salt. Discard the garlic and thyme; reserve the oil.
Transfer 2 tablespoons of the reserved oil to a large skillet. Add the fennel, onion and red pepper and season with salt. Cover and cook over low heat, stirring, until the vegetables are softened, 10 minutes. Add the vinegar and cook until evaporated, 2 minutes. Scrape the vegetables into a bowl.
Wipe out the skillet. Add 2 tablespoons of the reserved oil and heat until shimmering. Add the blanched potatoes and cook over moderately high heat, stirring once or twice, until crisp, 5 minutes. Return all of the vegetables to the skillet and cook over moderately high heat, stirring, for 2 minutes. Season with salt. Transfer the vegetables to a bowl, garnish with fennel fronds and serve.