Preheat oven to 200°C. Toss the beef in celery salt and half the olive oil, then fry in a large heavy based saucepan for 10 minutes, until well-browned. Set aside. Add the onions, celery, garlic, thyme and remaining olive oil and cook for 5 minutes. Mix in the tomato paste and return the beef.
Pour in the Guinness and stock, then bring to a boil. Simmer gently with the lid slightly ajar for 3 hours, until the beef is tender. Strain the liquid into a second saucepan and boil until reduced to 1½ cups. Mix the flour and butter, stir in and simmer until thickened. Return to the beef mixture and refrigerate until cold.
Roll the mascarpone, sour cream and icing sugar in a bowl and whip gently to soft peaks. Spoon cherries and a little syrup into the bottom of four glasses and top with the mascarpone mix. Scrape the frozen granita with a fork and spoon on top. Garnish with baby mint and serve immediately.
Brush the tops with egg and bake for 35-40 minutes, until the pastry is deep golden and crisp.