Add lamb, rosemary, mint jelly, mint and garlic to sous vide bag and vacuum seal. Cook in sous vide for ~10-12 hours. For rare cook at ~128 degrees Fahrenheit, medium rare ~134 and medium ~138-140. After lamb has cooked, sear it on the grill. In a small saucepan over medium heat, heat butter until melted and stir in flour. Cook for a minute to reduce floury flavor. Add warmed water, juices from the sous vide bag, and beef base and cook until thickened, stirring constantly. Adjust seasoning if necessary.