Combine onions, water, and salt in a saucepan.

Cover and cook for 20 minutes or until crisp-tender; drain.

Transfer onions to two greased 1-quart baking pans.

Sauté mushrooms in 4 tablespoons butter and lemon juice in a skillet.

Melt 4 tablespoons butter in a small saucepan; stir in flour until smooth.

Gradually add milk, stirring continuously.

Bring to a boil, then cook and stir for 2 minutes until thickened.

Stir the mushroom mixture into the thickened sauce.

Pour the mushroom sauce evenly over the onions in each baking pan.

Melt the remaining butter.

Toss together cheese and breadcrumbs; then add melted butter and toss to coat.

Sprinkle the breadcrumb mixture over each casserole.

Cover and freeze for up to 3 months if desired.

If frozen, thaw in the refrigerator for 8 hours before baking.

Cover and bake in a 375-degree oven for 15 minutes.

Uncover and bake for an additional 10 minutes, or until golden brown.