Heat the olive oil in a heavy-bottomed pot over medium heat.

Add the beef cubes in batches and brown them on all sides. Remove from pot and set aside.

In the same pot, add onions and sauté until translucent, about 5 minutes. Stir in garlic and cook for another minute.

Add tomato paste, stirring well to incorporate and cook for 1 minute.

Return the beef to the pot, pour in stout beer (if using), and deglaze the pot.

Add beef broth, carrots, potatoes, thyme, rosemary, bay leaf, salt, and pepper.

Bring to a gentle boil, then reduce heat to a simmer, covering partially. Cook for 1.5 to 2 hours.

Stir occasionally, ensuring it doesn't stick to the bottom; cook until beef is fork-tender.

Boil chopped potatoes in salted water until tender, about 15 to 20 minutes. Drain.

Return potatoes to pot over low heat for a few minutes to evaporate excess moisture.

Mash potatoes with butter and cream until smooth; season with salt and pepper.

Remove bay leaf from the stew, taste for seasoning, and serve hot in bowls topped with fresh parsley and mashed potatoes on the side.