Make herb butter: In a bowl, combine soft butter, parsley, garlic, salt and pepper. Mix thoroughly.
Place on cling film and roll neatly into a firm roll. To simplify slicing, place in the freezer for 30 minutes, along with the venison filet. (Which will need to be half-frozen and firm to be sliced.)
On the kitchen table, next to your cutting board, spread out another layer of cling film. Cut the half frozen venison filet in paper-thin slices.
Arrange on cling film, creating a thin layer of meat. Slice the herb butter using a mandolin or a sharp knife. Arrange a thin layer of herb butter on top of the meat.
Holding on to the edge of the cling film, fold the layers together twice (removing the cling film from the meat as you fold it the second time). Leave in the fridge until serving. Just before serving, cut the terrine in 1 by ½ -inch pieces and arrange on toast.