Mustard and horseradish sounds good to me.
Another easy one would be chili sauce from a jar or try this easy recipe.
Pantry Chili Sauce
1 one pound can of whole peeled tomatoes. (This is now a 14 ½ ounce can. You can use diced, crushed, stewed etc…)
1/3 cup of vinegar (White or Apple Cider)
1/3 cup of light brown sugar
1/3 cup each of finely diced celery, onion, and green pepper.
½ t each of salt, ginger, cinnamon, allspice.
¼ each of black pepper and ground cloves.
Put tomatoes into a saucepan and break them up if they are whole or in large pieces, add all other ingredients and bring to a boil. Reduce heat and simmer uncovered for 30 to 40 minutes, stirring every 10 minutes or so. Using a stick blender or food processor blend until the texture is a coarse puree. Cool and refrigerate. This recipe yields one pint.
You could also get a little fancy with a Cumberland sauce or a horseradish cream sauce. Here are a couple of examples.
Cumberland Sauce | Delia Online
Horseradish Cream Sauce Recipe - Allrecipes.com