Prick the sirloin steak with a long, sharp kitchen knife and insert the bacon fat in the holes. Season the meat well with salt and pepper all over and add the allspice and peppercorns. Place half of the diced vegetables in a large enough dish, place the meat on top and cover with the rest of the vegetables. Melt the butter in a frying pan and pour over everything, cover with a lid or plastic wrap and place it in the fridge to chill overnight.
The next day, melt a knob of butter in a large skillet and place the meat inside and sear it on all sides over high heat.
Place the meat in a roasting pan together with all the vegetables, pour the beef stock over it and cover with aluminum foil or a lid.
Place in the middle rack of a preheated oven at 320° F (160° C) for 1-2 hours, or until the meat becomes very tender. Baste it about every 30 minutes or so so it doesn't dry up. While the meat is roasting we can prepare the dumplings.
When the meat is done, transfer the meat to a separate dish and leave it to rest and cool slightly. Meanwhile, mash the vegetables using a hand blender and place them in a saucepan.
Pour in the cream and bring the mixture to a boil. Then stir in the vinegar, lemon, sugar and salt to taste and cook for 5-10 minutes until nicely thickened. If the sauce is still a little runny, add a little flour to thicken it up and continue cooking for another 10-15 minutes until glossy.
Once the steak is cool enough to handle, slice it into 1/2-inch slices and place them in the pan with the sauce to heat through.
Serve the svíčková along with the dumplings. If you like you can also add a little cranberry jam on the side and a dollop of whipped cream.