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Chop the kale into about 1" size pieces. Add kale to boiling salted water and cook for about 10 minutes. Drain and set aside.
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In a large pan, gently sauté onion and bell pepper in oil until soft and translucent. Add garlic and cook until fragrant. Add the cooked kale to the onion/pepper mixture and set aside until you have make your sauce.
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To make the sauce: melt the butter in a pan, stir in flour, and cook for a couple of minutes over gentle heat. Gradually whisk in milk to make a smooth lumpless sauce and stir until thickened. Turn off the heat, stir in half the grated cheeses. Season, with pepper, nutmeg and a little salt if needed.
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Swipe a gratin dish with smidge of butter. Stir most of the sauce into the kale/onion mixture. Turn into a buttered gratin dish, spreading the last of the sauce on top.
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Mix cheeses with the breadcrumbs and strew them on top of the dish. Warm kale gratin through in a 370 F oven, about 20 minutes. Before serving, trickle cream around the inner rim of the gratin dish. Pass briefly under a hot broiler (watching) to finish crisping and browning the top.