Image from Bon Appetit
1 medium onion
5 garlic cloves, divided
2 cups heavy cream
1 rosemary sprig
1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, plus more
½ tsp. freshly ground black pepper, plus more
4 medium or 3 large bunches of Tuscan kale and/or Swiss chard (2–2½ lb. total)
1 lemon
¼ tsp. crushed red pepper flakes
6oz. high-quality sharp cheddar cheese
½ cup panko (Japanese breadcrumbs)
1 tsp. extra-virgin olive oil
Preheat oven to 350°. First, prep the cream sauce: Peel and quarter 1 medium onion. Smash 4 garlic cloves and unpeel. Place onion and garlic in a medium saucepan. Pour in 2 cups heavy cream and stir in 1 rosemary sprig, 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, and ½ tsp. freshly ground black pepper. Bring to a simmer over low heat and cook, stirring occasionally, until onion and garlic are very soft, 15–20 minutes.
While cream cooks, remove ribs from 4 medium bunches of Tuscan kale. Tear leaves into big pieces and wash. It’s okay if some water still clings to the leaves—this will help them wilt later. If you don’t have a bowl big enough to hold all of your greens, pile them on a baking sheet instead.
When cream is ready, pluck out rosemary sprig and discard. Finely grate zest of ½ lemon into cream and stir in ¼ tsp. crushed red pepper flakes. Transfer to a blender and blend until smooth.
Heat a large (as large as possible—12" is the goal) ovenproof skillet over medium. Add about one-quarter of greens, cover skillet (use a baking sheet if you don’t have a lid), and cook until wilted, 3–4 minutes. Scoot wilted greens to one side of pan. Add another quarter of greens, cover pan, and cook to wilt, 3–4 minutes. Remove from heat and place skillet on a rimmed baking sheet.
Prep the toppings: Cut 6 oz. high-quality sharp cheddar cheese into ¼"-thick slices (you’ll have about 1½ cups). Place ½ cup panko to a small bowl and finely grate in zest of ½ lemon and remaining 1 garlic clove. Add 1 tsp. extra-virgin olive oil, season with salt and pepper, and stir to combine.
Pile remaining greens on top of wilted kale, pressing down to compact as needed. This will look BANANAS, but the greens will shrink dramatically in the oven. Carefully pour cream mixture over. It’s okay if it drips a little—that’s why it’s on a baking sheet. Break cheese slices into large crumbles and evenly distribute over top. Sprinkle panko mixture over.
Bake gratin until cheese is browned and bubbling and panko is toasted, 50–55 minutes.
Let cool 5 minutes before serving. Try not to eat the whole thing yourself!