Blanch the julienned leeks and carrots in salted water until tender. Drain, cool, and reserve.
Heat 10 cl goose fat in a pan. Add diced porcini mushrooms and fennel, season with salt and pepper, and sauté for 5 minutes, stirring often. Transfer to a cold plate to cool.
In a bowl, toss potato slices with chopped bay leaf, salt, and pepper.
Line a terrine mold with pork belly slices and reserved vegetable julienne. Layer ingredients in this order: potatoes, foie gras slices, potatoes, porcini-fennel mixture, potatoes, foie gras slices, potatoes. Top with remaining pork belly.
Bake the terrine in a bain-marie at 120°C for 2.5 hours. Remove from oven, weight down, and refrigerate for 2 days.
For the salad, sauté porcini slices in a Teflon pan with 10 cl goose fat. Season, add chopped shallots and bacon cut into 2 cm pieces. Transfer to a large platter to cool.
Slice terrine into 10 portions. Arrange slices around the mushrooms, add broccoli florets. Dress with vinaigrette made from shallots and wine vinegar. Garnish with chervil and parsley. Serve immediately.
