Reserve blood from a freshly killed carp; mix it with a little vinegar and set aside.
Clean and remove inside of carp, taking care not to damage the gall; wash thoroughly.
Cut fish into four pieces.
In a saucepan, mix beer with 2 cups (1/2 liter or about 1 pint) water.
Add onion, salt, peppercorns, bay leaf, lemon juice, sugar and butter.
Cook 20 minutes, stirring occasionally.
Strain liquid, return to saucepan and add carp blood; beat to blend well.
Stir in gingersnap crumbs.
Bring to boiling.
Put pieces of carp into sauce and cook over low heat about 30 minutes.
Arrange pieces of carp on a preheated platter.
Strain sauce again; season to taste and pour some of it over carp.