Preheat oven to 450 degrees F.

In bowl, add flour and salt. Using whisk, mix the water with the flour until lumps are gone. It will be loose like pancake batter. Cover and let stand for at least 15 minutes or up to 12 hours.

When batter is ready, add olive oil to 12 inch pizza pan or oven safe iron skillet (that's what I used). Spread the onions in the pan evenly and place in hot oven for a few minutes. Remove from oven and pour batter over onions. Sprinkle with rosemary (or mix into batter before pouring).

Bake for about 30 minutes until edges are a bit charred and the top is nicely browned.

Remove from pan and enjoy right away or store for a couple days and heat before serving.