In a large mixing bowl, combine the sugar, yeast, and warm water. Stir well and leave in a warm place for 15 minutes, or until the mixture becomes foamy and bubbly. Whisk in the Greek yogurt, olive oil, and salt until combined.
Add the flour, garlic, and coriander. Mix with a fork until a shaggy dough forms.
Turn the dough out onto a lightly floured surface and knead gently for 2–3 minutes until smooth. If the dough is sticky, add a little extra flour as needed.
Divide the dough into 10 equal pieces and shape each into a ball. Lightly dust with flour, cover, and leave to prove in a warm place for 15 minutes.
Heat a large frying pan over medium-high heat. Roll one piece of dough into a circle approximately 6–7 inches (15–18cm) in diameter. Brush one side lightly with olive oil and sprinkle with a little salt. Place the flatbread in the hot pan, oil-side down. Brush the top side with olive oil. Cook for 1–1½ minutes, until the surface is covered with bubbles, then flip and cook for a further 1–1½ minutes until golden and cooked through. Repeat with the remaining dough pieces to make 10 flatbreads. Serve warm.