Here's a fun party trick: Ask your friends to guess how many pounds of greens went into this supremely cheesy gratin inspired by a recipe from chef Renee Erickson. Three huge bunches of kale (and/or chard) get drowned in garlic- and rosemary-infused cream, then blanketed in cheddar cheese and panko and sent into a hot oven, where they sizzle and shrink. The cream turns into a rich sauce and the cheese gets bubbly-brown; the greens towards the top burnish (kale chips!) but those underneath stay silky-soft. So. Many. Textures! You'll want to use your largest oven-safe skillet for this recipe. But don't worry: If you don't have a skillet that's at least 10" (and preferably 12"), you can wilt down the greens in whatever skillet you do have, then transfer them an oven-safe baking dish and proceed with the recipe.
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!