Activate the yeast. Mix active dry yeast, lukewarm water, and sugar. Wait until it is foamy, which should take about 5 minutes. In the mean time, measure flour.
Mix flour with salt and the yeast mixture. Mix with a rubber or wooden spoon until a sticky ball of dough forms. Add 1 tbsp of olive oil to easily bring it together. Cover with a dish towel and let it rise in a warm place until it doubles in size, so about 1 hour.
Preheat the oven to 425 F.
Grease or line a 10-inch cast-iron skillet. Pour the dough in. Drizzle 1 tbsp of olive oil on top an use your fingers to stretch the dough so it fills the skillet. Then dimple the dough. Top with onions and potatoes. Bake for 20-22 minutes, until the focaccia is nice and golden.
While the focaccia bakes, heat ¼ cup olive oil in a small pan over medium heat. Once you are sure the oil is hot, increase heat to medium-high and add mustard seeds. They should pop and sputter immediately. Toss in curry leaves and remove the pan from heat.
When the focaccia is done, let it cool before serving. Top with the spiced oil.