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Preheat the oven to 400 degrees F. Grease a 9 x 13 x 3-inch casserole dish and set aside.
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Place the parsley or celery root, or potatoes, in a large pot filled with cold, salted water. Bring to a boil and boil for 15 - 20 minutes, or until fork tender. Drain. Mash parsley root with 4 tablespoons butter and 1 teaspoon sea salt. Set aside.
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Meanwhile, heat a large skillet over medium-high heat. Melt 4 tablespoons butter, and add red onion. Cook 10 minutes, or until onions are translucent and have taken on some sweetness. Next add ground beef and a teaspoon of sea salt, and cook 4 minutes. Add carrots, celery, Worcestershire sauce, mustard seed, bay leaves, and pepper, and cook over medium heat for 8 minutes. Add kale and cook 2 minutes more. Check salt levels and adjust as needed.
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Spoon filling into prepared pan. Cover with vegetable mash; use the tines of a fork or a spoon to add texture to the surface. Bake 40 - 45 minutes, or until peaks have turned a rich golden brown and filling is bubbling vigorously.
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Cool 10 minutes, slice, and serve.