In a large(ish) roasting pan, combine the tomato paste, garlic, thyme, demiglace (if using), Guinness and beef stock. Whisk this together breaking up the tomato paste and demiglace into the liquid. Season with salt and pepper.
Place the vegetables into the roasting pan. Then season both sides of the pot roast and place it on top of the vegetables. If you r roasting pan has a cover, use it, if not cover the entire pan with foil. Braise in the oven for about 3 ½ hours. Check after 3 hours, the meat should pull apart with no effort and the fat should be basically melted off the beef. When meat is totally tender remove from oven and cool until you can handle it.
Remove beef from roasting pan and pull apart, some larger chunk and some shredded. Strain the sauce, pouring through a colander to catch all the vegetables, and into a fat separator. If you don't have one of these, just pour into a bowl and skim the fat off the top, reserving at least four tablespoons. Take the vegetables and roughly chop what you can, the carrots will likely fall apart and that is totally fine.
In a large saucepan, pour four tablespoons of reserved fat and heat over medium heat. Add four tablespoons of flour and whisk until bubbly and the raw flour smell is gone, it should smell nutty. Continuing over medium heat, slowly pour the strained beer and stock into the flour mixture, whisking constantly to combine. This will thicken and become basically a gravy.
Turn the oven to 400.
Back in your roasting pan, put the beef, veggies and gravy and stir to combine. Here you can either spoon mixture into individual baking dishes (liek the pictures) or you can use a large baking dish or even the just keep in the roasting pan. Whatever you do cover each with puff pastry to fit to the rim of the baking dish. You can brush the top of the puff pastry here with heavy cream or an egg yolk for a nice golden crust.
Bake in the oven until the puff pastry has puffed, is brown and the filling is bubbling around the edges.
