Melt the butter in a small saucepan with the rosemary sprig; keep warm.
Place wine, lemon juice, and shallot in a small saucepan. Season with salt. Bring to a boil, reduce heat, and simmer briskly for approximately 7-8 minutes, or until liquid has reduced to about 2 tablespoons; it will look syrupy. Scoop out and discard the shallot quarters. Add saffron powder and stir to blend. Set aside the reduced liquid.
Brush the sweet potato slices on both sides with the rosemary butter and place on a parchment-lined baking sheet. Salt the top side of the sweet potatoes. Roast for 10 minutes, checking at 8 minutes; do not let the potatoes brown.
Pat the fish fillets dry and season well on both sides with salt and white pepper. Coat them in flour. Heat 1 tablespoon of the butter in a 10-inch skillet. When melted, and the foam has subsided, add the fish fillets and cook until golden brown. Carefully turn the fillets to cook the other side. Once the second side is golden, the fish should be perfectly cooked. Keep warm.
Place the reduced wine and citrus on medium heat and start whisking in the remaining cold butter, 1 tablespoon at a time, until creamy.
Arrange 4 sweet potato slices on each plate. Top with a fish fillet, and then spoon the beurre blanc over top. Garnish each fillet with a sprig of rosemary and serve.
Notes
You can substitute halibut or sea bass for the black cod.