Combine 1 Tbsp. Sriracha Ranch Dip Mix, oil and lemon juice in a gallon resealable bag; massage to combine. Add pork slices; seal bag and toss to coat. Refrigerate several hours or overnight.
Combine remaining Sriracha Ranch Dip Mix with sour cream and mayonnaise. Store in refrigerator, covered, until ready to serve.
Heat large non-stick skillet over medium-high heat. Remove pork from marinade and add to skillet. Sauté until browned on all sides and internal temperature reaches 165°.
Serve pork in toasted bun with Sriracha Ranch Dip and lettuce, tomatoes and onion, if desired.