Cook onions in simmering, salted water for 5 minutes, then drain. Heat olive oil in a small saute pan, add onions and sugar, toss to coat. Cook, tossing occasionally, until onions are tender and golden brown.
In a large bowl mix the ground lamb with the flour, beaten egg, ground nutmeg and salt and pepper. Separate the mixture into 4 equal patties. On a hot grill or barbecue grill patties for approximately 5 minutes on each side (for a medium rare burger) or until desired degree of doneness.
After cutting the focaccia into rounds, slice each round in half. Lay the grilled lamburgers on top of focaccia. Crumble the feta over patties. Top with caramelized onions (cut in half), a drizzle of olive oil and fresh mint leaves. Replace remaining rounds of focaccia.
Suggested Drink: Banrock Station Southeast Australia Shiraz, 1997