Cook broccoli until just tender-crisp.
Remove heads and tails from trout, if you wish.
Sprinkle trout with lemon juice and let stand 15 minutes.
Simmer in heated wine or chicken stock until almost tender, about 5 minutes.
Remove trout; arrange with broccoli in a shallow baking dish.
If mushrooms are small, leave whole; cut larger ones in halves or quarters.
Saute in butter until lightly browned.
Add flour and mix until well blended.
Gradually stir in cream and 1/2 cup of the poaching liquid.
Cook, stirring constantly, until mixture boils and thickens.
Reduce heat and stir in salt and cheese until blended.
Spoon over fish in baking dish.
Bake, uncovered, in a moderately hot oven (375°) for 20 to 25 minutes.