Thaw the puff pastry in the refrigerator overnight.
Preheat the oven to 400°F Heat the canola oil in a wide skillet over medium heat. When it shimmers, add the onions, sugar, and salt. Sauté, stirring often, until the onions are golden and soft, 8-10 minutes. While they are cooking, add 1 tablespoon of water whenever the bottom of the pot gets brown. When they are ready, drain any extra oil or liquid and transfer the onions to a bowl. Stir in mustard.
Cut the puff pastry in 1 1/2-inch strips (a pizza wheel is excellent for this - use a ruler to keep the lines straight), then cut each strip in half the short way. Spread some of the onion-mustard mixture on each strip, and roll each strip around 1 piece of hot dog. Place seam-side-down on a rimmed baking sheet, leaving space for the pastry to puff. Repeat with the remaining pastry, onion mixture, and hot dogs.
Use a pastry brush to lightly brush each piece with egg wash. Sprinkle with poppy seeds and a touch of fleur de sel. Bake until puffed, golden, and crisp, 22 to 25 minutes. Serve right out of the oven.