BEETS AND CARAMELIZED ONIONS WITH FETA

Makes 4 first-course or side-dish servings.
This dish is a particularly good accompaniment to beef or lamb. (Active time: 20 min Start to finish: 45 min)

2 T cider vinegar
1/4 t black pepper
5 T olive oil
2 lbs. cooked beets, quartered (or halved if small)
1/4 cup pine nuts, toasted and coarsely chopped
1 t coarse-grain Dijon mustard
3/4 teaspoon salt
1 lb onions, 1/4d, cut crosswise into 1" pieces
3 oz crumbled feta (1/2 cup)

Whisk together vinegar, mustard, pepper, and 1/2 teaspoon salt in a large bowl, then add 3 tablespoons oil in a  slow stream, whisking until combined well. Cook onions with remaining 1/4 teaspoon salt in remaining 2 tablespoons oil in a 12-inch heavy skillet over moderate heat, stirring occasionally, until golden brown, 18 to 20 minutes. Add onions to dressing, then add beets and cheese, stirring gently to combine. Serve sprinkled with pine nuts.