Heat the oven to 400°F. Wilt the spinach in a large dry skillet over medium heat in batches, about 3 minutes. Cool, then squeeze in a clean towel until very dry and chop.
Melt the butter in the same skillet. Cook the shallot and garlic for 2 minutes until soft and fragrant.
In a large bowl, stir together the spinach, garlic mixture, artichokes, Greek yogurt, cream cheese, mozzarella, 1/2 cup of the parmesan, lemon zest, lemon juice, red pepper flakes, salt, pepper and nutmeg.
Spread into a 1-quart baking dish or an 8-inch cast iron skillet and top with the remaining parmesan.
Bake 20 to 22 minutes until bubbling at the edges, then broil 2 minutes until the top is golden. Rest 5 minutes and serve with pepper strips and sourdough.