Line a 6-quart slow cooker insert with a large piece of foil so it goes about 2 inches up the side. Spray generously with cooking spray.
Heat 1 teaspoon of the oil in a large skillet over medium-high heat. Add the shallots, and cook, stirring frequently, until they turn lightly golden, about 1 minute. Remove from the heat, and stir in the lemon zest and a pinch of salt.
Whisk together the eggs, egg yolks, half-and-half, 1 teaspoon salt and a few grinds of pepper in a large bowl.
Transfer the cooked shallots to a medium bowl. Add the ham, fontina and cornstarch, and toss. Scatter the mixture over the bottom of the lined insert. Pour in the egg mixture. Cover the slow cooker, and cook on high until the middle is set and no longer jiggles, about 3 1/2 hours. Remove the lid, dab the quiche with a paper towel to absorb any excess liquid and let rest for 20 minutes.
Meanwhile, wipe out the large skillet and heat the remaining 2 teaspoons oil over medium heat. Add a few handfuls of spinach, and cook, tossing and stirring, until wilted; repeat until all the spinach is wilted, about 8 minutes total. Season with 1/2 teaspoon salt a few grinds of pepper.
Use the foil to help remove the quiche from the insert. Use metal spatulas to lift it from the foil and onto a serving platter. Top with the wilted spinach, and scatter the Parmesan over the top. Serve warm or at room temperature.
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