Combine mayonnaise, buttermilk, Sriracha, and lime juice in a small bowl; cover and chill.
Place panko in an even layer on a baking sheet. Bake at 375° for 4 minutes or until golden. Transfer panko to shallow dish; cool completely. Add thyme, garlic powder, pepper, and salt; toss well. Place flour in a shallow dish. Place egg whites in another shallow dish. Dredge half of green beans in flour. Dip in egg whites; dredge in panko mixture. Place beans on a baking sheet. Repeat procedure with remaining beans, flour, egg whites, and panko mixture. Place remaining coated beans on another baking sheet. Bake green beans at 375° for 8 minutes or until crisp. Serve with Sriracha dip.