Slice the onion thin with a mandoline and soak them in water for a few minutes
In a large bowl, whisk together the flour and water to form a batter
Add the onion, breadcrumbs, parmigiano, rosemary, salt, and pepper. Mix well
Line a baking sheet with parchment paper. Drizzle with olive oil and sprinkle with semolina
Spread the batter evenly. Drizzle more olive oil and sprinkle with semolina on top
Bake at 400°F for 45 minutes, then broil until crispy and golden
