Marinate the fish in the lemon juice and olive oil.
Put a little olive oil in a pan, sautee the onions and mushrooms. Season with salt and pepper, and whichever spices you want. I used some paprika and all purpose seasoning.
Sautee till mushrooms shrink up and onions soften. Put aside.
Put a little olive oil in a non-stick pan, and when it's quite hot add the fish. Season the fish. Again use whichever spices you want. I used adobo seasoning and some rosemary. Brown on both sides and pour the marinating juices over it.
Cook down for about 3-5 mins until the fish in cooked. The take the fish out.
Put the mushroom mixture back into the pan and add the baby spinach. The liquid from the fish will help to wilt the spinach.
When the spinach has wilted plate it and put the fish on top.