Directions

1. Pierce the potatoes a few times with a fork. Microwave until softened but not completely cooked through, 3 to 5 minutes. Let cool slightly, then peel and finely chop; transfer to a medium bowl and set aside.
2. Bring a large pot of salted water to a boil. Add the spinach and Swiss chard leaves and cook until wilted, about 3 minutes. Drain, then let cool in the colander, tossing occasionally and lightly pressing with a rubber spatula to remove the excess liquid. When cool enough to handle, transfer the greens to a clean kitchen towel and squeeze out as much liquid as possible. Coarsely chop and add to the bowl with the potatoes.
3. Heat the olive oil in a large skillet over medium-high heat. Add the Swiss chard stems, onion, red pepper flakes, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring, until the vegetables are tender, about 8 minutes. Add the garlic and the spinach-chard mixture; cook, stirring, until well incorporated, 5 minutes. Transfer to a large bowl and let cool to room temperature. Add the cheeses, mint, dill, egg and lemon zest; stir to combine. (You can make the filling 1 day ahead; cover and refrigerate until ready to assemble.)