For the French toast: Preheat the oven to 325 degrees F. Place a wire rack inside a rimmed baking sheet. Lay the bread slices on the rack; set aside to air-dry. Whisk the cream, eggs, Parmesan, nutmeg and some salt and pepper in a large casserole dish until smooth; set aside.
For the Florentine sauce: Melt the butter in a small saucepan over medium heat. Whisk in the flour until the mixture is smooth and lump-free. Cook, whisking constantly, until light brown, about 1 minute. Slowly whisk in the milk and 1/4 cup of the heavy cream. Continue to cook, whisking, until the mixture has thickened, about 1 minute. Reduce the heat to low, stir in the spinach, 1/2 cup of the Parmesan, 1/4 teaspoon salt, a few grinds of pepper and the nutmeg. Turn off the heat, and cover to keep warm.
Heat a large nonstick skillet over medium-low heat. Transfer the bread slices to a plate. Transfer the baking sheet with the wire rack to the oven. Place 2 bread slices in the egg mixture, and let them completely soak on both sides, about 30 seconds to 1 minute per side, checking after 30 seconds. (The bread should feel heavy and soaked through but hold its shape without breaking apart.) Melt 1 tablespoon of the butter in the skillet. Add the 2 soaked slices, and cook until the outside is golden brown and crisp and the inside is cooked through, 3 to 4 minutes per side. You will need to watch and adjust the heat between medium and medium-low if the bread is browning too quickly or not browning quickly enough. Keep the finished French toast slices warm on the rack in the baking sheet in the oven. Repeat with the remaining slices.
Toss the remaining 2 tablespoons grated Parmesan with the parsley in a small bowl; set aside. Place the Florentine sauce back over medium-low heat, whisk in the remaining 2 tablespoons cream and the lemon juice and heat through, about 1 minute. Arrange 2 slices of French toast per plate, and divide the Florentine sauce evenly over the toasts (a generous 1/4 cup per serving). Garnish with a tablespoon of the Parmesan-parsley mixture and a few grinds of pepper, and serve.
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