Preheat oven to 400°F. Cut four 12" x 12" squares of cooking parchment; fold each in half to crease, then unfold. Place on work surface. Place 1 fillet immediately to the right of the fold, centered between top and bottom. Top each fillet with 1/4 of the salsa, 2 Tbsp. Sauerkraut, and 1 Tbsp. olives. Fold unfilled side of parchment over filling, matching edges. Starting with edge opposite fold, fold 1/4" over and crease; then fold and crease again to make a double fold. Fold and crease the other two edges to make double folds. Repeat folding and creasing to make another round of double-fold seals. Place packets on baking sheets. Bake 10 minutes for every inch of fish thickness or until packets puff up and brown lightly, and fish is cooked through. Place packets on plate and serve immediately. Slit or tear open each packet at the table, being careful to avoid steam burns. Top each with 2 tsp. sour cream.
Cook's Tip: Enjoy as-is, or use the parchment's contents to fill taco shells for Zesty Fish Tacos.