Place beets in a large pot; add a small layer of water just to cover. Bring to a boil; cook until water evaporates and beets are fork tender when pricked. If the water evaporates before they cook, add another ½ cup (125 mL).
Add apples, onion, vinegar, ginger, maple syrup, raisins and mustard seeds; reduce heat to medium. Cook for about 20 to 25 minutes, mashing beets and apples with a potato masher until soft. Season with salt and pepper.
Nutrients per serving: 69 calories, 0 g fat (0 g saturated fat, 0 g trans fat), 0 mg cholesterol, 34 mg sodium, 16 g carbohydrates (1 g fibre, 13 g sugar), 1 g protein