Preheat oven to 375 degrees F. If beet greens are attached, remove all but 1/2 inch of the stems (so as not to pierce the beets) and reserve for another use. Put beets in a baking dish with 1/4 cup water. Cover tightly and bake until a small knife slips in easily, 40 to 45 minutes. Cool slightly, then peel. Let cool completely, then cut into 1/2 inch wedges.

In a small bowl, whisk together 4 teaspoons vinegar, garlic, olive oil, and salt and pepper to taste.

In a small bowl, toss together beets and red onion with about half of the dressing. Let stand 10 minutes to absorb flavors, then taste and sprinkle with a little more vinegar if necessary.

In a large bowl, toss spinach with remaining dressing. Taste and adjust seasoning. Divide equally among 4 plates. Top with beets and onions, again dividing equally. Crumble feta evenly over each salad.