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1
Preheat oven to 400u00b0 F. Get an ice bath ready in a large heat-proof bowl.
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2
Separate long stems from beet greens. Wash all greens thoroughly (seriously, look for the dirt!) and place in a colander.
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3
Blanch greens in a large pot of salted boiling water in batches, starting with beet stems or a tougher green like collards or larger leaves and cook accordingly (roughly 2 minutes for stems or tougher leaves, about a minute for smaller, more tender leaves). Remove stems or greens with tongs in batches and plunge into ice bath. Repeat until all greens are blanched, adding more ice to the ice bath if necessary.
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4
Heat olive oil over medium heat in a 10-inch skillet. Add onion and a pinch of salt and saute for a minute or two before adding garlic. Stir and cook for another minute before adding diced apple. Cook a few minutes more until onion is translucent, apple starts to release juices and mixture is fragrant.
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5
Add vermouth and cook, stirring until liquid is reduced by half, about 2 minutes.
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6
Add Dijon, Worcestershire, and heavy cream, stirring between additions. After the cream, taste for salt and adjust if necessary. Add a few grinds of black pepper. Remove pan from heat.
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7
While apple-cream mixture is cooling, with clean hands, remove greens by the handful from ice bath and thoroughly squeeze out water. Squeeze beet stems separately. Chop squeezed greens and beet stems.
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8
Ladle enough apple-onion-cream mixture into a 9-inch glass or ceramic pie plate to just cover the bottom.
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9
Arrange chopped greens in pie plate, using fingers to slightly fluff and unfurl compacted greens. Evenly spread out apple-onion-cream mixture (solids first and top entire mixture with liquid) on top of greens. Top with chopped walnuts and finally gruyere cheese. Press down on walnuts and cheese with the back of a spoon.
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10
Place pie dish on a baking sheet and bake gratin for 20 to 22 minutes, until bubbling and top is golden. Allow gratin to cool a bit before serving. Enjoy!
