Step 1

Preparation Pour reserved mussel cooking liquid through a fine sieve lined with a dampened paper towel into a bowl. Set aside 24 mussels in their shells, then shuck remainder and halve them crosswise.

Step 2

Wash chopped leeks in a bowl of cold water, then lift out and drain well.

Step 3

Cook leeks, carrots, bell peppers, and shallot with salt and pepper in butter and oil in a 5-quart heavy pot over moderate heat, covered, stirring occasionally, until tender, about 7 minutes. Stir in garlic and cook, uncovered, stirring, 1 minute. Add strained mussel cooking liquid and wine and simmer 10 minutes.

Step 4

Stir in halved mussels and cream and simmer, stirring, 5 minutes. Add reserved mussels in their shells and simmer until just heated through, about 1 minute. Season with salt and pepper.