Nutrition per serving may change if servings are adjusted.
2 teaspoons canola oil
1 cup chopped onion
6 cloves garlic, minced
2 (5 ounce) packages baby kale
¾ cup whole milk, divided
2 tablespoons all-purpose flour
2 ounces Gruyère cheese, grated ( ½ cup)
¾ teaspoon salt
½ teaspoon ground cumin
¼ teaspoon ground nutmeg
⅛ teaspoon cayenne pepper
4 cups very thinly sliced butternut squash (about 1¼ lbs.)
1 (1 ounce) slice stale bread (see Tip)
1 teaspoon butter
Preheat oven to 375°F. Coat an 8-inch-square baking dish with cooking spray.
Heat oil in a large nonstick skillet over medium-high heat. Add onion and garlic; cook, stirring often, until tender and golden, about 6 minutes. Stir in kale by the handful; cook, stirring frequently, until wilted, about 5 minutes.
Whisk 2 Tbsp. milk and flour in a small bowl. Heat the remaining milk in a small saucepan over medium heat until little bubbles form on the surface, about 3 minutes. Whisk the flour mixture into the hot milk; cook, stirring constantly, until slightly thickened, 1 to 2 minutes. Whisk in cheese, salt, cumin, nutmeg, and cayenne; cook, stirring constantly, until the cheese is melted and the sauce is thickened, 1 to 2 minutes. Stir the cheese sauce into the kale mixture.
Layer half of the squash in the prepared baking dish; top with half of the kale mixture. Repeat with the remaining squash and kale mixture. Cover with foil.
Bake the gratin until the squash pierces easily with a fork, 40 to 50 minutes.
Meanwhile, place bread in a food processor and process until it forms coarse crumbs. Melt butter in a large nonstick skillet over medium-high heat. Add the breadcrumbs; cook, stirring, until toasted, 5 to 7 minutes. Transfer to a small bowl.
Sprinkle the gratin with the toasted breadcrumbs and let stand for 20 minutes before serving.
Tip: If you don't have stale bread, bake a slice at 275°F until dry to the touch, 8 to 10 minutes.
To make ahead: Peel and slice the squash up to 2 days ahead and refrigerate.