Preheat the oven to 200C/392 F (fan)
Start by cutting the red onions and bell peppers. Slice the onions thinly and cut the bell pepper into small strips.
Add the bell pepper and onion to a bowl, and add dry herbs, salt and pepper and plain flour and corn meal. Mix to combine, it will remain dry, then slowly add water, the mixture should remain thick. It will look like it won't hold together (because the bell pepper and onion are thick) but that's fine. It should look like there are way more vegetables than flour water mixture. Now we spread it on a baking tray on an oiled baking paper and press it down thinly. These quantities can either fit on big tray or two medium ones. Make sure it is thin and even, otherwise some parts could burn while the rest still needs to cook.
Place baking paper on a baking tray, add some olive oil and spread it all over the paper. Drizzle with olive oil, corn meal, and salt.
Bake until crispy, for 50-60 minutes until golden brown on top. To get the crunchy bottom, move the tray at the bottom of the oven for the last 10 minutes. If the top is getting burnt while the center is still not ready, cover it with some baking paper. It won't be hard and crispy immediately after taking it out. If you touch the top of the scarpaccia with your finger it should still feel slightly soft, that's normal, it will harden when cooling down. Let it cool down for 15 minutes before slicing.