Preheat oven to 400 degrees F. Put a large pot of water on to boil. Coat an 8-inch-square baking dish with cooking spray.Advertisement
Cook kohlrabi in the boiling water until tender, about 10 minutes. Drain.
Meanwhile, melt butter in a small skillet over medium heat. Add leek and cook, stirring occasionally, until softened but not browned, about 5 minutes. Remove from heat.
Heat 1 cup milk in a small saucepan over medium heat until steaming. Whisk flour and the remaining 1/3 cup milk in a small bowl to make a smooth paste; stir into the hot milk and cook, whisking constantly, until the sauce thickens, 2 to 3 minutes. Remove from heat and stir in salt, pepper and nutmeg.
Arrange 1/3 of the kohlrabi in the prepared pan. Top with half the leek and 1/3 cup each cream sauce and cheese. Repeat with another layer of kohlrabi, the remaining leek and 1/3 cup each cream sauce and cheese. Top with the remaining kohlrabi and sauce. Sprinkle with the remaining 1/3 cup cheese and breadcrumbs.
Bake until bubbling and golden on top, 25 to 30 minutes. Let cool for 5 minutes before serving.
To make ahead: Prepare through Step 5; cover and refrigerate for up to 1 day.
2 1/2 vegetable, 1/2 fat, 1/2 high-fat protein, 1/2 starch