Pat the beef cubes dry with a towel to remove excess moisture, then generously season them with freshly ground black pepper. Heat the olive oil in a large heavy-bottomed pot or Dutch oven over medium-high heat. Brown the beef in batches, ensuring not to overcrowd the pot to achieve a proper sear on all sides. Once browned, transfer the beef to a plate.

Add a splash of beef broth to the pot, scraping up the browned bits from the bottom. Add the chopped carrots, celery, and onion, stirring occasionally until they soften, about 5 minutes. Stir in the garlic and cook for another minute until fragrant. Pour in the red wine, bringing it to a simmer and allowing it to reduce by half, about 4 minutes, while scraping up any remaining brown bits from the bottom.

In a separate bowl, whisk the flour into the remaining beef broth until smooth. Pour this mixture into the pot, along with the sugar. Return the browned beef to the pot, bring the mixture to a boil, then reduce to a simmer. Cover and cook for up to 2 hours, or until the beef is tender and can be easily pierced with a fork.

During the last 20 minutes of cooking, add the baby potatoes and whole grain mustard. Adjust seasoning with salt and pepper as needed. Serve the stew warm, garnished with freshly chopped parsley if desired. This dish pairs wonderfully with rice or rustic bread, and the leftovers are equally delightful.