Add the chickpeas flour and the water in a bowl and stir with a whisk until well combined. Let the batter rest for at least one hour.
Slice the onions and cut the potatoes into small cubes. Stir fry them until soften and golden brown. Add salt to taste.
Transfer the onions and the potatoes on a baking tin lined with parchment paper. Pour the chickpeas batter in the tin. The onion and the potatoes must be completely covered with the batter.
Drizzle some oil on the top of the farinilla and cook in the oven at 220° for 40 minutes. Let the farinilla cool down a bit before cutting it.