Caldeirada is a fish soup/stew very popular in Portugal, which has several recipes (mainly suggesting different kind of fish). It is cooked in a large(r) pot and these recipes have all in common to start covering the pot with a layer of sliced onion rings. Then you add layers of other vegetables and the last layers would be the fish. You are not supposed to stir while cooking.
Why is the first layer always onions? Is this just a question of adding sufficient onion flavor to the dish or is there another reason?
some recipe's excerpts and respective links (all in Portuguese)
https://pt.petitchef.com/receitas/prato-principal/caldeirada-fid-1512802 - In a reinforced deep pan arrange the ingredients in alternate layers, starting with a layer of onions followed by garlic, tomatoes, peppers, potatoes and fish
https://www.pingodoce.pt/receitas/caldeirada-de-peixe/ - Step 3: Pour half the olive oil into a large pan and layer the vegetables on top, starting with the onion.
https://www.vaqueiro.pt/recipes/caldeirada-a-moda-de-peniche-198125 -Peel the onion, cut them into thin slices and spread them over the bottom of the pan. Peel the potatoes, cut them into rounds and place them on top of the onions. Wash and cut the peppers into strips taking care to remove all the seeds and white skin. Spread the pepper strips over the potatoes. Wash the tomatoes, cut them into pieces and arrange them on top of the remaining vegetables.