Cook the spinach according to package instructions. Drain well and squeeze out any excess water.
Heat oil and butter in a large saute pan (ovenproof is good).
Add the minced onion and mushrooms. Saute till softened.
Add the spinach and stir well till heated through. Add salt and pepper to taste. If using the same pan, separate spinach into the same amounts as you have fish. Usually 4 or 5 sections if the fish is small. Or you can use a pyrex 9 X 18 oven proof dish. Place fish over spinach and sprinkle with paprika. Bake till fish flakes easily. About 30 minutes. Squeeze lemon juice over and serve.