Pan-Roasted Halibut with Mushrooms & Lemon Butter Sauce

Ingredients for 2 portions:
1 tbsp olive oil
1 tbsp butter, melted in oil
2 (7-oz each) halibut filets
salt and cayenne to taste
handful of clamshell mushrooms, or sliced un-cool, regular mushrooms
1/4 cup water
juice from 1/2 lemon, or to taste
1 tbsp chopped Italian parsley
1 1/2 tbsp butter to finish sauce