Bring water and vinegar to a boil in a Dutch oven. Reduce heat; add fillets. Cover and simmer 1 minute or until opaque. Remove from liquid; discard liquid.
Coat a nonstick skillet with cooking spray; add 1 tablespoon margarine. Place over medium-high heat until margarine melts. Add spinach; saute 1 minute.
Stir flour into 1/2 cup broth. Bring remaining 1/4 cup broth and milk to a boil. Add flour mixture; cook, stirring constantly, until thickened. Add 1/4 cup Swiss cheese, sour cream, lemon juice, 1/2 teaspoon garlic, salt, and pepper. Stir 1/2 cup cheese sauce into spinach mixture. Spread in an 11- x 7- x 1 1/2-inch baking dish coated with cooking spray. Arrange fillets over spinach; pour remaining cheese sauce over fillets. Combine remaining 1/4 cup Swiss cheese and 1/2 teaspoon garlic, breadcrumbs, and remaining ingredients. Sprinkle over casserole. Bake at 375° for 15 minutes.