Add your 2 tablespoons of flour, then coat the onions and carrots in the flour.
Add your beef stock, water, seared lamb, potatoes, tomato paste, Worcestershire sauce, thyme and bay leaves. Bring the mixture to a boil, then down to a simmer. Let the mixture bubble away for about 20 minutes stirring occasionally so the mixture doesn't stick and burn at the bottom.
Once you have a nice, thick stewy mixture, season the filling to taste with salt and pepper, add your fresh mint and mix through.
Pre-heat your oven to 180 degrees C (360 degrees F)
Lightly dust a flat clean surface with your extra flour, then place one sheet of puff pastry on your surface. Place the other sheet directly on top, then dust the pastry lightly again. Roll the 2 sheets of pastry against one another until the pastry binds together and becomes as thin as one sheet.
If you want, you can use some of the excess pastry to create the shamrock symbol, egg wash it, then stick in on top of the pie. Egg wash all of the pastry on top so it comes out of the oven nice and golden brown.