Heat the olive oil in a large, heavy-bottomed pot over medium heat.
1 tablespoon olive oil
Pat the beef dry with paper towels and season generously with salt and freshly ground black pepper.
1 ½ pounds beef chuck, Salt and freshly ground black pepper
Brown the beef in batches, searing all sides until well browned. Remove the beef and set aside.
In the same pot, add the sliced onions, carrots, and bay leaves. Cook for 5 to 10 minutes, stirring often, until the onions are soft and translucent.
2 cups onions, 2 carrots, 3 bay leaves
Return the browned beef to the pot. Add the chopped tomatoes, Guinness beer, brown sugar, salt, and black pepper. Stir to combine.
1 can, 2 cups Guinness beer, 1 tablespoon brown sugar
Bring the stew to a gentle simmer. Cover and cook for 1½ hours, stirring occasionally.
While the stew simmers, melt the butter and mix it with the flour to form a smooth paste.
2 tablespoons unsalted butter, 2 tablespoons all-purpose flour
Stir the mushrooms and roux into the stew. Continue cooking for 15 minutes, or until the stew thickens.
2 cups mushrooms
Taste and adjust seasoning with more salt, pepper, or brown sugar if needed.
Remove the bay leaves and serve hot over creamy mashed potatoes.