DIRECTIONS

Preheat oven to 425°F.

Brush onions with 2 teaspoons vinegar in a lightly oiled shallow (1-inch-deep) baking pan. Roast in middle of oven, turning over halfway through roasting and keeping wedges intact, until browned and tender, about 15 minutes. Cover to keep warm.

Reduce temperature to 400°F.

Coarsely grind fennel seeds, salt, and pepper to taste with a mortar and pestle or in an electric coffee/spice grinder, then spread on a plate. Roll pork in fennel mixture, coating all sides evenly.

Roast in a flameproof small shallow roasting pan in middle of oven until a thermometer inserted 2 inches into center of pork registers 155°F, 40 to45 minutes. Transfer pork to a plate and let stand, covered, 15 minutes.

Add broth to roasting pan and deglaze by boiling over moderately high heat, scraping up brown bits, 1 minute. Transfer broth to a small saucepan and simmer until reduced to about 1/2 cup, 10 to 15 minutes. Add remaining teaspoon vinegar and simmer 1 minute. Stir together cornstarch and water and stir into broth. Bring to a boil, then remove from heat and stir in butter.

Serve pork with onions and sauce.