Baked Spinach Casserole

3 boxes (10 ounces each) frozen chopped spinach1 tablespoon butter1 large onion, chopped 1/2 cup mayonnaise1 cup sour cream2 cans (about 11 ounces each) cream of mushroom soup1 tablespoon lemon juice1 cup or less shredded Monterey jack or cheddar cheeseCoarse breadcrumbsHeat oven to 350 degrees. Coat a 2-quart casserole with nonstick spray. Microwave spinach and drain very well. Cool and wring out thoroughly in paper towels. Place in large bowl.Melt butter in sauté pan and cook onion until tender. Stir into spinach in bowl. Add mayonnaise, sour cream, mushroom soup and lemon juice; stir well. Spoon into prepared pan and sprinkle cheese on top, followed by breadcrumbs. Bake 35-45 minutes or until bubbly and browned.Serves eight.Cook's notes: Can be prepared one to two days in advance. Bring to room temperature before baking, or allow for longer cooking time.Nutrition values per serving: 394 calories, 30 g fat (11 g saturated), 23 g carbohydrates, 4 g fiber, 11 g protein, 40 mg cholesterol, 890 mg sodium.