Make the dough: In a mixing bowl, combine the flour, yeast, sugar, and salt. Gradually add warm water until you have a soft, elastic dough.
Rest the dough: Cover with a cloth and let it rise in a warm place for about 1–1.5 hours, until doubled in size.
Heat the oil: In a deep pan, heat vegetable oil over medium-high heat until hot but not smoking.
Shape the doughnuts: Take small handfuls of dough, stretch them into rings, and carefully drop them into the hot oil.
Fry until golden: Fry for about 2–3 minutes per side, turning once, until puffed and golden brown.
Coat in sugar: Drain on paper towels, then immediately roll in sugar while still warm.
Serve: Eat fresh and hot — the Tunisian way!