Disolve sugar and yeast in warm water and let stand until foamy and proofed.
Combine flour and salt in large bowl.
Using your hands, rub butter into flour until it is absorbed.
Make well in the center of flour and pour in yeast and milk.
Still using hands,push flour from sides of bowl into center and mix to form dough.
Knead several minutes until smooth.
Place in lightly oiled bowl and turn to coat.
Cover with plastic wrap and let stand in warm area until doubled, about 1 hour.
Preheat oven to 450 degrees.
Set butterflied lamb skin side down on working surface.
Pound to flatten slightly.
Mix butter,garlic,salt and pepper to a paste; rub half thoroughly into lamb.
Spread with stuffing and reshape leg.
Sew closed with string;tie crosswise at 1 inch intervals and twice around length to hold stuffing.
Rub outside with remaining butter mixture, then rub with rosemary.
Set on rack in roasting pan and roast 20 minutes.
Reduce oven temperature to 350 and roast an additional 40 minutes.
Let cool, then remove string and brush off excess rosemary.
* Lamb can be prepared ahead to this point. Wrap in plastic wrap and refrigerate up to 24 hours.Don't start the pastry until you take the lamb out of the refrigerator.
Lightly oil shallow roasting pan.
Knead dough once again.
Roll out enough dough on lightly floured surface to make rectangle 1/4 inch thick, large enough to completely enclose lamb with a 1 inch overlap.
Set lamb on one end of rectangle.
Beat yolks with milk and brush some on edges of dough;fold dough over lamb.
Press seams together; trim and reserve excess dough.
Set seam side down in roasting pan and brush with some of yolk.
Preheat oven to 450 degrees. Roll excess dough into long thin ropes. Braid in threes to go around top perimeter of lamb.
Remaining dough can also be made into smaller and thinner ropes and/or worked into more intricate designs such as leaves,tiny birds, flowers and coiled snakes inside the encircling braid, all symbols of Easter in Greece.
Brush decorations with yolk mixture.
Bake until pastry sets, about 15 minutes.
Reduce oven temperature to 300 degrees and bake an additional 20 to 30 minutes(use a meat thermometer to check doneness).
Transfer to heated platter and garnish with fresh mint sprigs, if desired.