Preheat oven to 375°.

Pierce the potato in several places with fork. Bake on a parchment-lined sheet tray until tender, about 1 hour. Let stand until just cool enough to handle, but still warm, about 15 minutes.

While the potatoes are baking, bring a large pot of water to a boil, and add a generous pinch of salt. Blanch the peas for about 30 seconds, and using a skimmer, transfer the peas to a bowl of ice water. Note: Save this water for boiling the gnocchi. Plus, you'll need 1/4 cup for preparing the pea puree.

Drain the peas, and place them in a blender with the 1/4 cup reserved water and lemon zest. Blend the pea mixture until completely smooth, adding more water as necessary if it's too thick. Set aside.

While the potatoes are still warm, remove the skin and pass them through a ricer or a food mill into a large bowl.

Add the pea puree and egg, and gently mix until just incorporated.

Add the flour, salt and pepper, and using a wooden spoon, very gently incorporate the flour into the dough. When the dough starts to come together, transfer the dough to a floured surface. Gently knead the dough until it just comes together. Note: The dough should be very slightly tacky, but if it's too wet add more flour.

Divide into 8 pieces and roll each piece into a rope about 1/2 inch thick. Cut the ropes into 1-inch pieces. Place the gnocchi on a parchment-lined and very well-floured sheet tray.

Cook in the boiling water until they float to the top, about 2 minutes. Test one to be sure they are cooked through.

Drain well and toss in the pan with Tarragon Cream Sauce. Tip: Add a splash of the gnocchi cooking water if the sauce is too thick.