Step 1Roast the bell peppers over an open flame or in the broiler, turning until blackened all over. Place them in a paper bag inside a plastic bag and tie it closed; leave the peppers to steam in the bag, about 20 minutes. Remove the blackened skin and the seeds and cores.
Step 2Place the peppers in a blender and puree until almost smooth. Add the garlic, serrano chile, egg yolk and season with one-fourth teaspoon of salt and one-fourth teaspoon of pepper. Puree again until smooth.
Step 3With the blender running, add the olive oil in a slow drizzle, mixing until emulsified. Taste and adjust the seasoning as desired. This makes about 2 cups of rouille. Cover and refrigerate up to 1 day if not using immediately.
Step 1Season both sides of the fillets with a pinch each of salt and pepper.
Step 2Heat a large saute pan over high heat until hot. Add enough oil so there is a thin film along the bottom of the pan and heat just until smoking. Place the fish fillets in the pan, careful not to crowd. (This may need to be done in two batches.) Sear the fillets quickly on each side just until cooked through, 1 to 5 minutes per side, depending on the type of fish and thickness of the fillets. Remove the cooked fillets to a large plate or platter.
Step 3Place about one-fourth cup rouille on each plate, and center each fillet on top of the rouille. Garnish with chopped chives and serve immediately.