In a skillet, lightly toast the slivered almonds until they release their fragrance (remove from heat before they turn golden). In a food processor (or plastic reclosable bags with a meat mallet), grind the almonds until just medium-fine.
In a small mixing bowl, whip the butter until really creamy. Add the ground almonds, Parsley, lemon zest, minced Garlic and lemon juice, mixing well to combine. Salt and pepper to taste. Freeze as a flat envelope in an airtight plastic freezer bag or in an ice cube tray (for final cube storage in an airtight freezer bag).