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 the remaining mussels and halve them crosswise.
Wash the chopped leeks in a bowl of cold water. Then lift out and drain well.
Cook leeks, carrots, bell peppers and shallot with salt and pepper in the 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 the strained mussel cooking liquid and wine and simmer 10 minutes.
Stir in halved mussels and cream and simmer, until just heated through, about 1 minute. Season with salt and pepper.