1. Combine first 4 ingredients in a large Dutch oven; add 4 cups water. Bring to a boil; reduce heat, and simmer 20 minutes. Strain mixture through a sieve into a bowl, reserving stock. Discard solids.
2. Pour 1/4 cup stock into a small saucepan (set remaining stock aside); bring to a boil over high heat. Boil 2 minutes or until stock is reduced to 2 tablespoons. Reduce heat to low. Combine crème fraîche, juice, and egg yolk in a small bowl, stirring with a whisk. Slowly pour egg mixture into reduced stock, stirring constantly with a whisk. Remove from heat; stir in butter, 1/4 teaspoon salt, and pepper. Set aside.
3. Pour remaining stock into a large skillet; bring to a simmer over medium-high heat.
4. Line a vegetable steamer with 1 bunch basil leaves; arrange fillets in a single layer over basil. Place steamer in skillet; cover and steam fillets 15 minutes or until fish flakes easily when tested with a fork. Carefully remove steamer from skillet. Sprinkle fillets with remaining 1/4 teaspoon salt; let stand 5 minutes. Serve with sauce. Garnish with basil leaves, if desired.