1 tablespoon unsalted butter, softened
1 lb mixed bones and heads of white-fleshed fish (such as Pacific cod, porgy, striped bass, turbot, or any combination of these), gills removed and bones and heads rinsed well
1 small onion, sliced (1 cup)
12 fresh parsley stems (reserve leaves for another use)
2 tablespoons fresh lemon juice
1/2 teaspoon salt
1/2 cup dry white wine
3 1/2 cups cold water

Spread butter over bottom of a 2- to 3-quart heavy saucepan, then add fish bones and heads, onion, parsley stems, lemon juice, and salt. Steam mixture, covered, over moderately high heat, 5 minutes.

Add wine and water and bring to a boil. Skim froth, then reduce heat to moderate and simmer, uncovered, 25 minutes.

Pour stock through a fine-mesh sieve into a large bowl, pressing hard on solids and then discarding them.

Cooks' note:
Stock can be made ahead and cooled completely, uncovered, then chilled, covered, 2 days or frozen 1 month.