Prick each sausage in several places with tip of small knife. Pour beer into bottom of pan fitted with steaming rack or basket, making sure beer doesn't cover rack, and heat beer to boiling. Adjust heat to maintain gentle boil and place sausages on rack. Steam covered until sausages are firm to the touch and just cooked through, 6-8 minutes. Remove from heat and transfer to paper towels.
About 10 minutes before serving, heat butter and oil in large heavy skillet. Pat sausages dry and place in skillet. Saute, turning with tongs, until well browned on all sides, about 10 minutes. Cut sausages diagonally into 3 or 4 pieces each. Serve hot with cornichons, onions, and mustard.