Step 1: In a large pot, settle 3 pounds of potatoes. Cover with cold, salted water by 1 inch. Bring to a rolling boil, turn down to a low boil, and cook until potatoes are easily pierced with a skewer, about 10 minutes. Drain.
Step 2: When potatoes are cool enough to handle (but still warm), halve them. Pile into a large bowl. Sprinkle in 1 cup peas (no need to defrost) and 3/4 cup red onion.
Step 3: Whisk together 1/2 cup mayo, 1/2 cup sour cream, 1/4 cup dill, 2 tablespoons mustard, 1/2 teaspoon lemon zest, and salt and pepper to taste. Pour this dressing over potatoes. Mix gently with a soft spatula.