Mixed Greens Gratin with Hazelnuts

Preheat oven to 350°F. Coat an 8-inch square baking dish with oil.

Prepare a large bowl of ice water. Bring a large pot of water to a boil over high. Add greens, return to a boil, and cook until tender, about 2 minutes. Drain greens, and plunge immediately into ice water. Once cool, drain well, and squeeze out excess water. Roughly chop greens. Set aside.

Melt butter in a large, deep skillet over medium. Add onion, and cook, stirring occasionally, until translucent, about 6 minutes. Sprinkle flour over onions, stir until evenly coated, and cook 1 minute. Whisk in milk, and simmer 2 minutes. Stir in 1/4 cup Parmesan, salt, orange zest, and pepper. Stir in chopped greens.

Pour mixture into prepared baking dish; top evenly with hazelnuts and remaining 1/4 cup Parmesan. Bake in preheated oven until golden brown and bubbling, 30 to 40 minutes.