A quick sauté renders baby kale leaves perfectly tender, allowing them to almost melt into the creamy béchamel.
Cook pasta according to package directions, adding 1 Tablespoon salt to boiling water. Drain; set aside.
Preheat oven to 350°F. Heat oil in a large (12-inch) ovenproof skillet over medium. Add kale by handfuls, stirring to wilt in between additions. Season with salt and teaspoon pepper. Place kale on a paper towel–lined plate to drain. Wipe skillet clean.
Melt butter in skillet over medium. Add flour, and cook, whisking constantly, until mixture is very lightly browned, 1 to 2 minutes. Gradually add milk and cream, whisking to combine, until completely incorporated. Cook over medium, whisking constantly, until mixture is smooth, thickened, and gently bubbling, 3 to 5 minutes. Remove from heat, and gradually whisk in cheddar until melted and smooth. Whisk in eggs; fold in cooked pasta and drained kale. Season with salt and pepper. Sprinkle evenly with cheddar cheese.
Bake in preheated oven until lightly brown and bubbling around the edges, 20 to 25 minutes. If desired, broil 1 to 2 minutes to brown the top. Let cool 10 minutes before serving.