Roll out the single-crust pie dough; press it into a 9-inch pie plate, tart pan or a springform pan with a removable bottom. Refrigerate until ready to fill. Preheat the oven to 350 degrees.
Bring a pot of salted water to a boil over high heat. Add the kale and cook for 30 seconds, then immediately drain the kale into a colander. Squeeze out as much water as possible. As soon as the kale is cool enough to handle coarsely chop it and set it aside; there should be 1 cup.
Add just enough oil to lightly coat the bottom of a medium skillet. Heat over medium-high heat, then add the diced onion. Cook for 2 to 3 minutes, until the onion softens. Add the chopped kale and a pinch of salt; cook for 1 to 4 minutes or until the excess moisture has evaporated. Remove from the heat, then add the dill and parsley, stirring to combine.
Whisk the eggs and egg yolk together in a large mixing bowl, then add the cream and stir to incorporate.
Combine the cornstarch and milk in a measuring cup, stirring to mix well, then add it to the egg-cream mixture in the bowl. Add pepper to taste and the nutmeg, whisking to combine.
Scatter the kale-onion mixture over the prepared pie dough, then evenly distribute the crumbled feta over the vegetable mixture. Pour the egg mixture on top.
Bake for 45-50 minutes, until the tart is set and the crust is lightly browned. Let it sit for 15 minutes before serving.