Make the avocado crema: In a blender or small food processor, combine the avocado, sour cream, lime juice, garlic, cilantro (or parsley, if using), and salt, and pulse until smooth. The consistency should be similar to sour cream – thick yet spreadable. To thin it, add a tablespoon or two of water and pulse to incorporate. Cover and set aside until ready to serve. The crema will keep in an airtight container in the refrigerator for 24 hours.
Prepare the salmon: In a small bowl, combine the sugar, chili powder, salt, and cayenne pepper. Lay the salmon filets on a clean work surface and sprinkle the brown sugar mixture evenly over each filet, rubbing it into the flesh on all sides.
In a 12-inch non-stick skillet set over medium-high heat, heat 2 teaspoons of the oil. Add two of the salmon filets (skin side up, if they have skin on) and cook, undisturbed, until their undersides are crisp and just beginning to blacken, about 4 minutes. Flip the filets and cook until the fish feels firm to the touch, 4 minutes more. Transfer to a warm plate. Add the remaining 2 teaspoons of oil to the pan and swirl to coat. Repeat the cooking process with the remaining 2 filets of salmon. Set the salmon aside.
Add the sliced lime rounds to the skillet and cook just until they begin to caramelize, about 30 seconds. Flip and cook for an additional 30 seconds.
To serve, divide the salmon filets among 4 plates and spoon 2 tablespoons of cream over each. Press a caramelized lime slice into the crema to garnish and serve.
Reprinted with permission from Eating in the Middle by Andie Mitchell (Clarkson Potter, 2016).