For lamb riblets:

• 8 lamb breast riblets (about 1 1/2 pounds total)
• 2 quarts lamb stock
• 1 large onion, chopped fine
• 1 garlic clove, halved lengthwise
• 1 tablespoon tomato paste
• 1 small bunch fresh thyme (about 8 sprigs)
• 3 bay leaves
• I tablespoon whole black peppercorns

For vegetables:

• 1 russet (baking) potato
• 2 large carrots
• 2 large turnips
• 2 medium zucchini
• 1 celery rib, cut diagonally into 1/4 inch slices

For vinaigrette:

• I tablespoon Dijon mustard
• 2 tablespoons drained bottled horseradish
• 1/2 teaspoon red-wine vinegar
• 2 teaspoons fresh thyme leaves, chopped fine
• 1/2 teaspoon freshly grated lemon zest
• 1/2 cup olive oil

For lamb loin:

• a 1 pound piece trimmed boneless lamb loin
• 1 tablespoon unsalted butter, softened
• 2 tablespoons canola or vegetable oil
• 1/4 cup fresh oregano leaves
• 1 1/2 tablespoons olive oil
• freshly ground white pepper to taste

Make lamb riblets:

In a large dry nonstick skillet brown riblets over moderate heat until golden brown on all sides, 6 to 8 minutes, and transfer to a large heavy kettle. Stir in remaining riblet ingredients and simmer, uncovered, stirring occasionally, 1 1/2 hours. Transfer riblets with metal tongs to a plate. Pour mixture remaining in kettle through a fine sieve into another large heavy kettle, discarding solids. Boil stock mixture, uncovered, stirring occasionally, until reduced to about 2 cups, about 30 minutes, and keep warm. Riblets and reduced stock mixture may be made I day ahead, cooled completely, and chilled separately, covered.

Make vegetables;

Peel potato, carrots, and turnips and cut into 1 1/2 by 1/2 inch pieces. Bring a 3-quart saucepan of salted water to a boil. Cook potatoes, carrots, and turnips until knife-tender, about 5 minutes, and transfer with a slotted spoon to a plate to cool in one layer.

Trim unpeeled zucchini and cut into 1 1/2 by 1/2 inch pieces. Have ready a bowl of ice and cold water. Return water in pan to a boil and cook zucchini and celery until knife-tender, about 3 minutes. Transfer zucchini and celery with a slotted spoon to bowl of ice water to cool completely and drain in a colander.

Make vinaigrette:

In a bowl whisk together mustard, horseradish, vinegar, thyme, zest, and salt and pepper to taste and whisk in oil in a slow stream until emulsified. Vinaigrette may be made I day ahead and chilled, covered. Bring vinaigrette to room temperature and whisk before proceeding.

Make lamb loin:

Preheat oven to 400' F.

Pat loin dry and season with salt and pepper. In a heavy ovenproof skillet heat butter and oil over moderately high heat until foam subsides and brown lamb, turning it, on all sides, about 2 minutes.

Roast lamb in middle of oven about 7 minutes for medium-rare, or until a meat thermometer inserted in middle registers 140 F. Transfer lamb to a cutting board and let stand 20 minutes.

Whisk vinaigrette into reduced stock mixture and add riblets and vegetables. Bring mixture to a boil, uncovered, and simmer 4 minutes. Cut lamb loin crosswise into 8 slices and arrange 2 slices in each of 4 shallow soup bowls. Top each serving with 2 riblets and spoon some sauce and vegetables around meat.

Serve pot-au-feu sprinkled with oregano, olive oil, and white pepper.