In a casserole over medium heat, heat bacon until all fat is rendered then remove bacon. Cook garlic and onions in bacon fat until translucent. Remove and set aside.
Brown the venison, add stock, bacon and onion then bring to a slow simmer. Allow to simmer for 1 hour. Add the diced vegetable and simmer until tender, about 15 minutes.
Remove from heat and stir in the Guinness. If gravy is not thick enough, thicken with a bit of cornflour. Cover with the sheet of pastry, poke holes to vent then bake at 180 degrees C for 10-15 minutes.