Fill a medium saucepan ¾ full with water. Place over high heat and bring to a boil. Place red onion slices in boiling water for 30 seconds and then drain. Pour remaining ingredients over onions and let sit covered at least 4 hours. Remove from liquid and serve as desired.
To make samba mustard
Mix Dijon, sambal, and mayonnaise well in small bowl. Refrigerate until ready to use.
To assemble sandwiches
Spread slider buns with butter. Toast until golden brown. Spread with sliced turkey, then provolone cheese, sliced tomatoes, pickled red onion, banana peppers, cilantro, and mixed greens.