Preheat the oven to 325˚F. Place the oil in a Dutch oven or other ovenproof pot set over medium-high heat. When the oil is hot, season the beef with salt and pepper. Cook the pieces of beef, in batches, until nicely seared.
Remove the cooked pieces of beef from the pot as you go along and set on a plate.
When the beef is all seared, add the onion, celery and garlic to the pot and cook four minutes. Mix in the flour and cook 2 to 3 minutes more.
Slowly, stirring constantly, mix in the stock. Mix in the mustard and rosemary and bring to a simmer. Cover and cook in the oven for 45 minutes.
Uncover the stew and stir in the squash, sweet potato and parsnip. Cover and cook in the oven 30 minutes more, or until the meat and vegetables are tender. Sprinkle servings of the stew with parsley, if using.