Chop the onion finely and heat in a little oil in a pot. When the onion becomes transparent, add the red lentils, salt, and about 250 ml (1 cup) of water. If all the water evaporates before the lentils are cooked, add a little more water.
When the lentils are cooked, add the spinach, stir 2-3 times and remove from the heat.
Clean the fish and season with salt and pepper, drizzle with olive oil, then place in a preheated oven at 180 °C (356 °F) for about 25 minutes.