Cut asparagus spears lengthwise in half. Then, cut into 2-inch lengths. Cook asparagus, covered, in lightly salted boiling water for 2 minutes. Drain. Quickly chill in ice water; drain again and set aside.
Bring broth to boiling in a 2-quart saucepan; reduce heat. Simmer, covered, while preparing risotto.
Melt ¼ cup butter in a 4-quart saucepan over medium heat. Add onions. Cook for 3 to 5 minutes or until onions are softened, stirring occasionally. Add rice to onions in saucepan; cook for 3 to 5 minutes or until rice is golden brown.
Remove saucepan from heat; add wine. Return saucepan to heat and cook until wine evaporates. Carefully stir ½ cup hot broth into rice mixture. Cook over medium heat until liquid is absorbed, stirring frequently. Continue adding broth, ½ cup at a time, and stirring frequently until the broth is absorbed each time. Stir in remaining ¼ cup butter, ½ cup cheese and asparagus; heat through.
Heat oil in a heavy skillet over medium-high heat. Add scallops and sear about 4 minutes or until opaque, turning once halfway through.
Divide risotto among four serving plates. Top with scallops. Garnish with lemon zest and additional Parmesan cheese, if desired. Squeeze lemon wedges over scallops before serving. Season scallops to taste with salt and black pepper.
