In a sterilized 4-ounce canning jar, mix together the yellow mustard seeds, brown mustard seeds, balsamic vinegar, and water. Cover and allow to sit at room temperature for 2 days.
Toss the beet quarters with the pepper, olive oil, and 1/4 teaspoon of the salt and roast at 375 degrees for 35 minutes on a baking sheet lined with tin foil. In a blender or food processor, blend one of the beet quarters with the mustard, brown sugar, and remaining 1/4 teaspoon of salt until pureed but still slightly coarse.
Use within 1 month. Makes about 4 Ounces (1/2 Cup) mustard.
Note: Will need to sit for multiple days before ready