Step 1Cook the pasta and cucumbers ahead to allow time for chilling before they are tossed with the dressing. That way, the flavors are not absorbed by the pasta.
Step 2Cook the pasta in boiling salted water according to the package instructions.
Step 3While the pasta cooks, split the cucumber lengthwise, remove the seeds and dice it small. Just before draining the pasta, stir in the cucumber to blanch it. Drain the pasta and cucumber and return them to the pot. Stir in the oil. Chill the pasta completely, at least 4 hours or overnight.
Step 4Add the onions, peas, mint, balsamic and wine vinegars, mayonnaise, buttermilk, 1 1/2 teaspoons salt and pepper to taste. Toss until the pasta is well-coated. Taste; adjust seasoning. (This can be mixed several hours ahead or overnight and refrigerated, covered airtight.)
Step 5To serve, add the lettuce; toss until mixed. Let rest 10 minutes before serving.