In a bowl, stir together the garlic–caper paste with remaining ingredients; season with salt and pepper to taste.
Chill mustard butter 10 minutes. Using wax paper as an aid, shape into a 6-inch log and wrap. Chill until firm, at least 1 hour and up to 3 days. Butter may be frozen, wrapped well, for 1 month. Slice as needed.
Notes
Yields about 3/4 cup. Serve on grilled or broiled chicken, meat, or fish, or toss with cooked vegetables.