Preheat the oven to 325 degrees F. Season the beef with salt and pepper. Heat the oil in a Dutch oven or large oven-proof pot set over medium-high heat. Cook the beef until nicely browned. Add the leeks and garlic to the pot and cook 3-4 minutes. Sprinkle in the flour and mix well. Slowly, stirring constantly, mix in the stock. Mix in the mustard, bay leaf and rosemary; bring to a simmer. Cover and cook in the oven for 45 minutes. Stir in the potatoes and carrots and cover and cook 30 minutes more, or until the vegetables and meat are tender. Adjust seasoning and serve.
Options: Leave the potatoes out of the recipe and serve the stew on top of mashed potatoes or egg noodles. Replace or substitute a portion of the vegetables called for with others that appeal, such as cubes of turnip, parsnips or squash.