Brown lamb, garlic, onions and thyme, in butter in large Dutch oven or heavy saucepan with tight fitting cover. Add stout beer (or wine), lamb stock, tomato paste, salt, pepper. Cover and simmer 45 minutes. Skim off any excess fat.
Add peas, carrots and mushrooms. Cover and simmer an additional 15 minutes, or until lamb and vegetables are tender.
Whisk flour into cream. Blend into simmering stew and bring back to a low boil. Cook 2 minutes, stirring until thickened. Taste and add additional salt and pepper if necessary.
Serve with mashed potatoes or place stew in soup bowls and top with a covering of mashed potatoes.