Melt 1/4 cup of the butter in a large frying pan over medium-high heat.
Add mushrooms and saute until limp.
Add onion, dill, bread crumbs, salt, and pepper. Stir until thoroughly mixed.
Add thawed and drained spinach, mix but do not cook spinach.
Spray shallow round cake pan with cooking spray (or a similar pan with low sides).
Spread spinach mixture over bottom.
Arrange fish on top of spinach.
Melt remaining butter and add lemon juice.
Brush fish with lemon butter and bake, uncovered, basting several times with more lemon butter until fish flakes when prodded in the thickest portion with a fork. Cooking time is approximately 10 minutes per inch of fish thickness.
Remove from oven and pour any remaining lemon butter over fish.