Start by preheating the oven 350 Fahrenheit or 180 degrees Celsius.
Peel the potatoes, wash them, cut them into thick slices, and add salt to taste.
On a separate pan, fry the potatoes. They don't need to be fully cooked, only half way as they will finish cooking in the oven.
We are going to prepare the "sofrito". A "sofrito" is the basic foundation of any Spanish stew:
On a frying pan, add the oil, and when the oil starts getting hot, add the chopped onions, garlic, peppers for approximately 15 minutes or until the onions are soft, and transparent.
Add the diced tomatoes, and Bay leaves, and cook for another 15 minutes or until the mixture starts to look like a tomato sauce.
Add the Sherry wine, and cook until the alcohol is fully evaporated.

On an oven-safe dish, arrange a bed of potatoes

Place the fish filets or pieces on top of the potatoes,

Cover with "the Sofrito".
Add salt and pepper to taste, and bake it for 40 minutes