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.
Cooking Time:Active time: 20 min Start to finish: 45 min
Ingredients:2 tablespoons cider vinegar
1 teaspoon Dijon mustard (preferably whole-grain or coarse-grain)
1/4 teaspoon black pepper
3/4 teaspoon salt
5 tablespoons olive oil
1 lb onions (2 medium), quartered lengthwise, then cut crosswise into 1-inch pieces
2 (15-oz) cans small whole beets, drained and quartered (or halved if very small)
3 oz crumbled feta (1/2 cup)
1/4 cup pine nuts (1 oz), toasted and coarsely chopped