1. Preheat oven to 400 degrees. On a lightly floured surface, roll out dough into a 10-inch round. Fit into an 8-inch tart pan with a removable bottom and fold over to make a double-thick edge. Roll rolling pin over the edge of the tart pan to trim the dough and pierce bottom of dough all over with a fork. Freeze until firm, about 15 minutes.

2. Line tart shell with parchment, and fill with pie weights or dried beans. Bake until crust is firm around the edges, about 20 minutes. Remove parchment and weights. Reduce oven to 375 degrees and continue baking until crust is pale golden, 10 to 15 minutes. Let cool on a wire rack.

3. While tart shell is baking, heat olive oil in a heavy saucepan over a medium-high heat. Cook onions, stirring frequently, until tender, about 3 minutes. Reserve 1/4 cup peas and add remaining peas to pan; cook 2 minutes more. Stir in milk and heavy cream and simmer over medium-low heat for 10 minutes. Season with salt and pepper and remove from heat. Let cool slightly before pureeing in a blender, then strain through a fine sieve; discard solids. Whisk together eggs and yolk in a large bowl, and gradually stir in pea puree.

4. Place tart shell on a rimmed baking sheet. Sprinkle with ham and reserved peas; pour in egg mixture. Bake until center is set and top is slightly puffed, 20 to 25 minutes. Transfer to a wire rack to cool before cutting into thin wedges. Serve warm or at room temperature.