Preheat the oven to 240°C/450°F/gas mark 8.
Put the chickpea flour into a medium mixing bowl. Slowly stir in the water, mixing continuously so it doesn’t get lumpy. It should be about the consistency of single cream.
Stir in 2 tablespoons olive oil.
Put in the fridge to chill for 1 hour.
Now there should be a thick bubbly layer on the top; scoop this top layer off with a metal spoon.
Put the remaining 3 tablespoons of olive oil into a 10 inch (25 cm) oven-proof skillet or frying pan. Let it heat for 5 minutes.
Now gently pour in the chickpea batter, it should sizzle vigorously so be careful.
While the pan is still hot, quickly and evenly scatter over the red onion slices, fresh rosemary, sea salt and put the farinata in the oven.
Cook for 11 minutes, then turn the oven to grill/broiler mode to grill the top for a minute or two, allowing it to brown a little more.
Ready to eat!