Cut a piece of foil twice as long as a large baking sheet. Loosely crumple the foil, and use it to line the baking sheet. Set aside.
Heat oil in a large skillet over medium-high heat. Add leeks; cook 4 minutes or until they begin to soften, stirring constantly. Cover, reduce heat to medium, and cook 8 minutes or until leeks are tender, stirring occasionally. Remove pan from heat, and stir in hazelnuts and next 4 ingredients.
Scrub oyster shells. Shuck oysters, discarding tops. Arrange bottoms on crumpled foil (to keep oysters from tipping over). Top oysters evenly with leek mixture.
Bake at 475°, in batches, 6 minutes or until oysters plump and edges bubble. Carefully arrange on a platter, and serve immediately.