- 2 pounds asparagus, trimmed
- 6 tablespoons (3/4 stick) unsalted butter
- 6 6- to 7-ounce cod fillets
- 2 teaspoons fennel seeds, coarsely chopped
- Cut asparagus tips into 2-inch-long pieces; set aside. Peel asparagus stalks. Cut stalks into 1-inch pieces. Cook stalks in large saucepan of boiling slightly salted water until very tender, about 10 minutes. Using slotted spoon, transfer stalks to blender. Add 3/4 cup cooking liquid to blender. Blend until very smooth, adding more cooking liquid to thin sauce. Blend 2 tablespoons butter into sauce. Season with salt and pepper. (Sauce can be made 8 hours ahead. Cover and refrigerate.)
- Transfer sauce to medium saucepan. Bring sauce to simmer. Remove from heat. Cover to keep warm. Cook asparagus tips in boiling salted water until crisp-tender, about 4 minutes. Drain.
- Sprinkle fish with salt and pepper. Melt 2 tablespoons butter in each of 2 heavy large skillets over medium-high heat. Add 3 fish fillets to each skillet and cook until golden and opaque in center, about 4 minutes per side.
- Spoon warm sauce onto plates. Top with fish. Garnish with asparagus tips. Sprinkle with fennel seeds and serve.