Place the flour, salt, and sugar in the bowl of a food processor fitted with the steel blade. Pulse for a few seconds to combine.
Preheat the oven to 400 degrees and line a sheet pan with parchment paper.
Peel, halve, and very thinly slice the onions into half moons, keeping the moons intact. Place the onion half moons on the pastry in diagonal lines, just barely overlapping and brush lightly with cream.
*Adapted from The Barefoot Constessa's French Apple Tart