In a large Dutch oven, heat olive oil over medium-high heat. Season the beef stew meat with salt and pepper, then brown on all sides for about 8-10 minutes. Remove and set aside.

In the same pot, reduce heat to medium. Add onions, garlic, carrots, and celery, sautéing for 5-7 minutes until onions are translucent.

Stir in tomato paste, Dijon mustard, and Worcestershire sauce. Cook for 1-2 minutes until combined.

Pour in apple cider and beef broth, scraping up brown bits. Add browned meat, thyme, rosemary, and bay leaf. Season with salt and pepper, cover, and simmer for 1.5 to 2 hours.

If desired, mix flour with water to create a slurry. Stir into simmering stew and cook uncovered for an additional 10-15 minutes.

While stew simmers, boil peeled and chopped potatoes in salted water for 15-20 minutes until fork-tender. Drain and return to pot to steam dry.

Add butter and milk to drained potatoes. Mash until smooth and creamy. Season with salt and pepper to taste.

To serve, place a scoop of mashed potatoes on a plate and ladle stew over the top. Garnish with fresh parsley if desired.