If the chard stems are very slim and tender, you can just sauté them along with the leaves. If they are thicker, prep them this way: Fold the leaves half lengthwise and slice along the edge of the center rib and stem to cut away the stems. Cut the stems crosswise into thin slices. Rinse the stems well in a colander and pat dry.
Stack a few leaves, roll them into loose cylinder, and cut or tear them into wide ribbons. Repeat with all the leaves. Rinse the leaves well in a colander and shake dry.
Heat 1/4 cup olive oil in a large skillet or Dutch oven over medium heat. Add the garlic and cook slowly to toast the garlic so it's soft, fragrant, and nicely golden brown--but not burnt-about 5 minutes. Add the chard stems, leeks, and chili flakes, and season with salt and black pepper. Sauté until the leeks are soft and fragrant and the chard stems are soft, 8 to 10 minutes.
Add the chard leaves; if they won't all fit in the pan, just add a few handfuls and toss them with tongs until they're wilted, and then add the rest. Add a splash of water and cover the pan. Cook over medium heat until the leaves are tender, 8 to 10 minutes. If there is a lot of liquid in the pan at this point, uncover the pan, raise the heat, and boil most of it off.
Transfer the vegetables to a bowl or platter to cool. Meanwhile, put the ricotta, Parmigiano, herbs, lemon zest, and eggs in a bowl and stir to blend well.
Add the cooled chard and fold together. Taste and adjust the seasoning with more salt, black pepper, or chili flakes.
Preheat oven to 350F.
Line a large baking sheet with parchment paper or a silicone baking mat. Lightly dust the work surface and roll out the dough to a rough 15-inch round. Roll the dough gently around your rolling pin, move it over the baking sheet, and gently unroll it into position, allowing it to drape over the sides of the sheet.
Pile the ricotta chard filling into the middle of the dough and gently spread it in an even layer to within about 3 inches of the edge. You should have at round of filling that's about 9 inches in diameter.
Gently fold the border of dough up and over the filling, pleating loosely as you work your way around the crostata. Don't aim for perfection.
Bake the crostata until the dough is nicely browned, on the underside as well as the edges, 25 to 30 minutes. Reduce the heat to 325°F and bake until the crust is cooked all the way through, another 15 to 20 minutes.
Carefully slide the parchment onto a rack and let the crostata cool for at least 15 minutes before cutting. You can serve it slightly warm or at room temperature.
