Pat beef cubes dry with paper towels. Season evenly with 1 tablespoon kosher salt and 1 teaspoon black pepper. Toss the seasoned cubes in 1/2 cup flour until lightly coated; shake off excess.

Brown the meat

Heat a large heavy-bottomed pot or Dutch oven over medium-high heat. Add 2 tablespoons vegetable oil. When shimmering, working in two batches so meat isn't crowded, add beef cubes and brown on all sides, about 3–4 minutes per batch. Remove browned beef to a plate and reserve.

Sweat aromatics

Reduce heat to medium. Add remaining 1 tablespoon vegetable oil and 1 tablespoon butter to the pot. Add diced onion, carrots, and celery. Season with remaining 1/2 tablespoon kosher salt and sauté until the onion is translucent and vegetables begin to soften, 6–8 minutes. Stir in minced garlic and cook 30–45 seconds until fragrant.

Build the stew base

Stir in 2 tablespoons tomato paste and 2 teaspoons flour (this second flour helps body without clumping). Cook, stirring, 1–2 minutes to remove raw flavor. Pour in 250 ml dark beer (if using) and scrape the bottom of the pot with a wooden spoon to loosen browned bits (deglaze).

Simmer with stock and herbs

Return browned beef and any accumulated juices to the pot. Add 900 ml beef stock, 2 teaspoons Worcestershire sauce, 2 bay leaves, and 3 thyme sprigs. Bring to a simmer, then reduce heat to low, cover partially, and simmer gently for 60 minutes, stirring occasionally.

Add potatoes and mushrooms

After 60 minutes, add halved potatoes and halved mushrooms to the stew. Stir, cover, and simmer until potatoes are tender and beef is fork-tender, about 25–35 more minutes. Skim any excess fat from the top as needed.

Finish and adjust seasoning

Remove bay leaves and thyme stems. Taste and adjust seasoning with remaining 1/2 tablespoon kosher salt and additional black pepper as desired. If the stew is too thin, increase heat and reduce uncovered 5–10 minutes, or mix 1 teaspoon flour with 1 tablespoon cold water to make a slurry, stir in and simmer 3–4 minutes to thicken. Stir in chopped parsley and the juice of 1/2 lemon (optional) to brighten the flavors.

To Serve

Ladle the stew into bowls. Serve with thick slices of rustic bread or sourdough and butter. Offer extra parsley and pepper at the table.