Cook onion in 2tbsp oil in a small heavy skillet over moderately low heat,stirring occasionally, until softened and beginning to brown 8 to 10 minutes. Cool 10 minutes.
While onions are cooling,stir together bread crumbs and milk in a large bowl and let stand until crumbs absorb milk.
Add onions and remaining ingredients to crumb mixture and stir with a fork until blended well.
Preheat oven to 250.
Form sausage mixture into 3 inch patties (about 1/2 inch thick) with dampened hands and arrange on a wax paper lined tray. Heat remaining 1/2 tsp oil in a 12 inch heavy skillet over moderately high heat until hot but not smoking, then cook patties in 3 batches, turning once, until browned and just cooked through, 4 to 6 minutes per batch. Drain patties briefly on paper towels as cooked, then transfer to a shallow baking pan and keep warm,covered with foil, in oven while cooking remaining batches.
Sausage patties can be formed (but not cooked) 1 day ahead and chilled,covered with plastic wrap.