Put softened butter, parsley, garlic and lemon rind in bowl, season with salt and pepper. Mix well.
Spoon along a sheet of baking paper, roll to form a sausage about 10cm long.
Chill until firm.
If using fresh chestnuts, carefully cut a slit in the shell of each chestnut with a sharp knife. Simmer for 10 minutes, peel immediately.
Cook carrots and Brussels sprouts for about 10 minutes in boiling salted water, or until tender, adding peeled chestnuts for final 5 minutes, and canned chestnuts for final 1 minute.
Drain well. Serve topped with slices of herb butter.