Heat oven to 450° F.
Roll the dough on a lightly floured surface to a 1/4-inch thickness. Sprinkle a baking sheet with cornmeal and place the dough on top.
Combine the leeks, potatoes, thyme, 3/4 teaspoon salt, 1/2 teaspoon pepper, and 1 1/2 tablespoons of the oil in a large bowl. Scatter the potato mixture over the dough and sprinkle with the Gruyère.
Bake until the crust is golden, about 20 minutes.
Slice into wedges and transfer to individual plates. Divide the greens among the plates. Sprinkle with 1/4 teaspoon salt and drizzle with the remaining oil.
Tip: To clean leeks, which tend to be gritty, slice them according to your recipe, then swirl in a bowl of cold water. The sand will sink to the bottom. Scoop them out and transfer to a colander to drain.