Bring a large pot of salted water to a boil. Add the shrimp and cook for 2-3 minutes until they turn pink and opaque. Drain and cool.

In a mixing bowl, combine the cooked shrimp, celery, green onions, mayonnaise, lemon juice, Dijon mustard, salt, and pepper. Mix well.

Slice the sub rolls in half lengthwise. Layer with shredded lettuce, tomato slices, and the shrimp filling. Close the rolls and serve immediately.