Preheat the oven to 400 degrees F. Butter a 9-by-13-inch baking dish with 1 tablespoon of the butter.
Scatter the turnips, radishes and celeriac on a baking sheet. Melt the remaining 1 tablespoon butter and drizzle over the vegetables. Sprinkle with the cumin, 1 teaspoon salt and some pepper and toss to coat the vegetables.
Roast, stirring once halfway through, until the vegetables are just tender and beginning to brown, about 12 minutes. Set aside to cool slightly. Reduce the oven temperature to 350 degrees F.
Meanwhile, add the chorizo to a large skillet over medium heat and cook, using a wooden spoon to break up any chunks, until cooked through, about 5 minutes. Transfer the chorizo to the prepared baking dish with a slotted spoon and spread in an even layer, leaving the fat from the chorizo in the skillet.
Return the skillet to medium heat. Add the onions and a pinch of salt and cook, scraping up any browned chorizo bits from the bottom of the skillet, until the onions begin to soften, about 3 minutes (see Cook's Note).
Scatter the onions on top of the chorizo in the baking dish and sprinkle half of the Monterey Jack over the onions. Add the spinach in an even layer on top of the cheese, then distribute the roasted vegetables on top of the spinach.
In a large bowl, whisk together the cream, eggs, 1 teaspoon salt and several grinds of pepper. Pour the egg mixture over top of everything in the baking dish. Top with the remaining cheese.
Bake until set in the center, 35 to 40 minutes. Let cool for 10 minutes. Top with sour cream before serving.
If the onions do not release enough liquid to bring up the browned chorizo bits, add up to 1/3 cup water a little bit at a time to help scrape them up. Just remember to cook off all the water before continuing.
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