Add mushrooms, onion and garlic to the pan and cook, stirring occasionally, until just starting to brown, 5 to 8 minutes. Sprinkle with flour and the remaining ¼ teaspoon salt and cook, stirring frequently, for 1 minute. Return the beef to the pan and stir in beer, broth and Worcestershire, scraping up any browned bits. Bring to a gentle simmer. Cover and cook until the beef and vegetables are very tender, about 20 minutes. Add peas and carrots, increase heat to medium and cook, stirring occasionally, until the stew is slightly thickened, about 5 minutes. Serve with the mashed potatoes, sprinkled with parsley, if desired.