Step 1

Position racks in upper and lower thirds of oven; preheat to 425°F.

Step 2

Toss beets, oil, salt and pepper together in a large oven-safe pot. Arrange the beets in a single layer in the pot. Combine water and vinegar in a measuring cup; pour over the beets. Cover the pot and place on the lower oven rack. Bake, flipping the beets halfway through, until very tender, about 1 hour. Remove from the oven and turn broiler to high.

Step 3

Transfer the beets to a large rimmed baking sheet, arranging them in a single layer. Press on each beet with the bottom of a mason jar or glass to flatten it, but keep it intact. Sprinkle with feta. Broil on the upper rack until the beets are crisped on the edges and the feta is browned in spots, 4 to 5 minutes. Sprinkle with oregano and serve.