Step 1Bring a large pot of salted water to a boil. Fill a large bowl with ice water. Submerge the snap peas and shelled peas in the boiling water. Return to a boil, then boil for 1 minute. Drain, then immediately transfer to the ice water. Let cool for 2 minutes, then drain again. Pat the peas dry with paper towels.
Step 2Thinly slice the snap peas at an angle and toss them in a large bowl with the shelled peas, pea shoots, mint, lime juice, olive oil and one-half teaspoon salt until evenly coated.
Step 3Spread the pipian dressing on a serving platter, mound the salad on top, and sprinkle with the pistachios.
Step 1Make the pipian: Place the tomatillos, onions, chiles, tarragon and pistachios in a medium saucepan and add enough cold water to cover by 1 inch. Bring to a boil, then reduce the heat to medium-low and simmer until the onions are tender but don’t yield completely when pierced with a fork, 25 to 30 minutes.
Step 2Reserve a quarter cup of the cooking water, then drain and transfer the solids to a blender. Add the spinach and salt and blend until smooth, adding cooking water by the tablespoon to loosen, if necessary. The sauce should be quite thick. Stir in the lime juice, then transfer to an airtight container and refrigerate until ready to serve.