1. Preheat the oven to 250°. Place the ribs on a long piece of aluminum foil (slightly longer than the rack of ribs) and season with ½ tablespoon salt and ½ tablespoon pepper. Fold the aluminum foil around the ribs so all the meat is covered. Place on a baking sheet and put in the oven. Bake until a paring knife easily slides into the meat, about 3 hours. Reduce the heat to 225°, and continue to cook the ribs until they are very tender and falling off the bone, about 2 hours.

2. When the ribs are nearly done cooking, start the fennel glaze: In a medium saucepan set over medium heat, add the red wine vinegar and the sherry vinegar. Cook until reduced by half, about 10 minutes.

3. While the vinegar reduces, caramelize the sugar: In a large skillet set over medium-low heat, evenly sprinkle ⅓ cup of the sugar. Cook the sugar without stirring until it begins to melt, then slowly sprinkle in the remaining sugar, cooking without stirring, until all of the sugar is dissolved. Once the sugar begins to turn golden, swirl the pan as needed to cook the sugar evenly until a uniformly golden caramel forms, about 7 minutes. Slowly pour in the reduced vinegar mixture (the caramel will hiss and bubble) and stir to combine. Stir in the ground fennel seed, ½ tablespoon black pepper, the red pepper flakes, lemon juice and 1½ tablespoons of salt. Remove the fennel glaze from the heat.

4. Remove the ribs from the oven, carefully open the foil and transfer the ribs to a baking sheet (pour any juices from the foil into the fennel glaze).

5. Increase the oven temperature to 400°. Use a pastry brush to coat both sides of the ribs with some of the fennel glaze, and place the ribs in the oven to warm the glaze, about 5 minutes. Remove the ribs from the oven and brush with additional glaze. Slice the rack into individual ribs and serve.