In a large pot or Dutch oven, heat olive oil over medium-high heat. Add the beef and season with salt and pepper. Brown the cubes on all sides, about 5 minutes.

Once browned, toss in the chopped onion and cook until soft, about 3-4 minutes. Then add minced garlic and mushrooms. Sauté until the mushrooms release their juice.

Stir in tomato paste, red wine (if using), and broth. Add the carrots, potatoes, thyme, and bay leaf.

Bring the stew to a boil, then reduce the heat to low. Cover it and let it simmer for about 1.5 to 2 hours.

While your stew is simmering, boil the potatoes in salted water for about 15 minutes or until fork-tender. Drain and return to the pot.

Add butter, milk, salt, and pepper to the potatoes. Mash until creamy and delightful.

Check on your stew; it should be thick and rich. Remove the bay leaf, taste for seasoning, and adjust if needed.

Serve the stew hot over a generous dollop of mashed potatoes.