Preheat oven to 400°. Toss beets with ¼ cup oil in a 13x9 inch baking dish; season with salt. Pour in water to barely cover bottom of pan. Cover tightly with foil (you want steam to build up in there) and roast beets until a paring knife slips easily through flesh, 60–75 minutes.

Let beets cool, then rub off skins with paper towels. Cut beets into irregular pieces (about 2 inches) and transfer to a medium bowl.

Using a vegetable peeler, remove two 2 inch long strips of zest from grapefruit (try not to get any of the white pith) and thinly slice zest lengthwise into strips; set aside for serving.

Cut grapefruit in half and squeeze juice over beets. Add shallots and vinegar, season generously with salt, and toss to coat. Let sit 15 minutes to allow shallots to soften slightly.

Heat ¼ cup oil in a small skillet over medium. Add rosemary and cook, stirring often, until sizzling subsides, about 15 seconds. Using a slotted spoon, transfer to paper towels and let drain; season with salt.

Arrange beets on a platter; drizzle with more oil and top with fried rosemary and reserved grapefruit zest.

Do Ahead: Beets can be roasted 2 days ahead. Remove skins; cover and chill. Bring to room temperature before cutting. Rosemary can be fried 3 hours ahead. Let sit at room temperature.