Shrimp and Snap-Pea Salad with Ginger Dressing

Coarse salt and ground pepper

sugar snap peas, stem ends removed (and, if necessary, strings)

peeled and deveined frozen medium shrimp, thawed

scallions, white and green parts separated and thinly sliced

vegetable oil, such as safflower

fresh lime juice

medium carrot, coarsely chopped

coarsely chopped peeled fresh ginger

large head Boston lettuce, torn into bite-size pieces

red bell peppers (ribs and seeds removed), thinly sliced