How to Make It
In a large bowl, whisk the vinegar with the shallot and 1/2 cup of the olive oil. Add the onion rings, season with salt and pepper and refrigerate for 1 hour.
Meanwhile, in a small bowl, whisk the mayonnaise with the lemon zest and juice. Gradually whisk in 1/4 cup of the olive oil and season with salt and pepper. Refrigerate the aioli until chilled.
In a large skillet, heat the remaining 1/2 cup of olive oil until shimmering. Spread the rice flour in a shallow bowl. Lightly dust the soft-shell crabs with rice flour and fry over high heat, turning once, until browned and crisp, 6 to 7 minutes. Drain the fried crabs on paper towels and season with salt and pepper.
Using a slotted spoon, transfer the marinated onion rings to a serving platter. Top with the fried soft-shell crabs and serve with the lemon aioli.
Rice flour is available at most supermarkets and at specialty and health food stores.