Heat oven to 400°F. Lightly oil potatoes, prick several times with a fork and sprinkle with salt. Place on lined baking sheet and bake until tender, 1 hour.
Meanwhile, place peas in small saucepan with 1 1⁄2 cups water to cover. Slice 4 scallions into 1-inch segments and add to peas. Finely chop remaining scallions and set aside.
Bring peas to a simmer over medium-high heat, remove from heat and drain, reserving 1⁄4 cup water. Place pea mixture in blender base with garlic, mint, 1⁄2 tsp. salt, and reserved cooking water. Puree until smooth.
Let potatoes cool slightly, peel and rice into a large bowl. Combine flour, potatoes, 1 cup of mixture, 2 tsp. salt and egg in a large mixing bowl. Mix in flour until fully incorporated and divide dough into eighths.
Lightly dust a clean work surface with flour. Roll each segment of dough into a log 10 inches long by 1 inch wide. Cut the log into 1″x1″ segments and roll each into a 3″x1⁄2″ rope. Cut each rope into 1″x1⁄2″ gnocchi and place on prepared tray. Rest uncovered at room temperature while preparing the remaining gnocchi, 15 to 20 minutes.
Add gnocchi to boiling water in two batches and cook 1 1⁄2 to 2 minutes, until gnocchi float on surface. Remove with a slotted spoon
Tarragon Cream Sauce
Meanwhile, bring a large pot of salted water to a boil. Place milk in a large skillet and bring to a simmer over medium heat. Reduce heat to low and simmer until reduced by half, 15 to 20 minutes. Stir in reserved scallion and chopped tarragon, and season with lemon juice, salt and white pepper to taste.
Add gnocchi to tarragon cream sauce and and toss so evenly coated. Garnish with reserved lemon zest and serve.