Trim top and bottom from oranges, then slice peel and pith off with vertical strokes. Slice oranges into 1/4-inch thick rounds.
In a medium-sized mixing bowl, use a fork to combine grapefruit zest, 1/2 teaspoon sea salt, and shallot. Mix in egg yolk. Use a whisk to beat egg yolk until it's pale and quite thick. Whisking constantly, whisk in olive oil a few drops at a time. Go very slowly until until about half the oil is added. Mixture should be thick, with soft peaks. If it's not thickening up properly, spoon mixture into another container, and begin again with a second yolk. Slowly add olive oil, and once it's thick and emulsified, whisk in grapefruit and yolk mixture. Whisk in all of the olive oil. Next, slowly whisk in 1 tablespoon grapefruit juice. Taste, and add more juice or sea salt as needed. Set aioli in fridge to chill until needed.