Wash and pat dry the pork. Cut off extra fat, but make sure to leave some. Don't discard the extra fat just yet, you might need to use it.
Mix spices in a small bowl. Use 1 teaspoon of salt for every 2 lb of meat. Add black ground pepper, paprika, dry thyme, sage, rosemary, basil, or whatever you like. Cut 4-5 garlic cloves into smaller pieces and dredge in spice mix. Make several shallow cuts in the meat and insert a garlic piece into each cut. If the meat is lean, insert the extra fat pieces as well. Rub the rest of the spice mix all over meat and brush with mustard. Put the meat into zip-locked bag or a container with a tight lid and refrigerate for at least 12 hours.
Take the meat out of the refrigerator and let it rest at room temperature for an hour. Preheat oven to 355F. Wrap the meat tightly in 2 layers of aluminum foil (make sure the shiny side faces out). Put into roast pan and pour about 1 inch hot water into the pan, roast for 1.5 hours. Check the water level and pour more if needed. Reduce temperature to 320F and roast for another 30 min.
Optionally: unwrap the meat, increase temperature to 350F, bast with juices and roast for another 10 min so it has a nice brown glaze. Wrap it back.
Turn off the heat, but leave the meat in the oven to cool down. Refrigerate. Serve cold, with rye bread and horseradish.