Fava Bean and Pea Salad with Poppy Seed Dressing

If using fresh fava beans, cook in a medium saucepan of boiling salted water until tender, about 4 minutes. Using a sieve, transfer to a colander set in a bowl of ice water. Drain well and peel; place in a medium bowl. (If using frozen fava beans, cook 2 minutes; do not peel.) Return water in saucepan to a boil; add peas and cook until tender, about 2 minutes. Drain; transfer to colander in ice water. Drain and add to bowl with fava beans. (If using frozen peas, cook 30 seconds.) Whisk sour cream, goat cheese, buttermilk, and lemon juice in a medium bowl, thinning with more buttermilk as needed, until pourable; season with salt and pepper. Arrange lettuce and pea shoots on a platter, top with fava beans and peas, drizzle with dressing, and sprinkle with poppy seeds. Do Ahead: Fava beans and peas can be cooked 1 day ahead. Cover and chill.