Caldeirada - the Portuguese fish soup

Question

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.

Answer 1

Tradition counts for a lot in many cuisines, so the answer may be 'because that's the way it's always been' in Portuguese cuisine. However, I would put the onion at the bottom because it takes the longest to cook. By putting the onions at the bottom they will get the earliest and therefore longest exposure to heat. Although you don't include the recipe, I would suspect that the layering starts with the longest to cook ingredients at the bottom and the shortest at the top, with the fish requiring the least cooking.