Preheat oven to 375 degrees F.
Brown lamb, a few pieces at a time, in 2 tablespoons of butter in a large heavy skillet over moderately high heat. Sprinkle last batch with sugar and let brown slightly, then transfer all to a Dutch oven.
Add flour and 2 tablespoons of butter in skillet to make a roux, stirring constantly until mixture is smooth and is just lightly browned, slowly stir in stock, tomato puree, 1 teaspoon salt and the pepper, and heat stirring until smooth; pour over meat, add bouquet garni and garlic, cover and bake 45 minutes.
After 45 minutes, add onions, carrots and potatoes, cover and bake 30 minutes or until vegetables are cooked but still firm.
Ladle into a deep platter and serve with crusty bread. Don't forget to take out the bouquet garni.