To prepare the fillets, combine yogurt, mayo, onions, and cilantro in a dish. Place fillets on a foil lined baking sheet, sprinkle with salt and pepper, then spread the mixture of the tops of the salmon.
Mix together the panko and melted butter (add some lemon zest and salt and pepper as well if you would like more flavor), the press the mixture onto the salmon. Bake at 425°F for about 20 minutes, or until fish flakes easily with fork.
For the sauce, combine all ingredients together. If want a thinner sauce, add a teaspoon or two of water. Serve sauce alongside salmon.