Whisk ½ cup cream, 1 tablespoon lemon juice, and 1 teaspoon salt in bowl. Combine chicken and cream mixture in large zipper-lock bag; refrigerate 30 minutes.
Heat oil in large skillet over medium-high heat until shimmering. Cook onion until golden, about 3 minutes. Add mushrooms, bell pepper, ¼ teaspoon salt, and pepper and cook until vegetables have softened, about 5 minutes. Stir in flour and cook 1 minute. Add Madeira, scraping up any browned bits with wooden spoon, and cook until thickened, about 1 minute. Add broth and remaining cream and cook until sauce is very thick and spatula leaves trail when dragged through sauce, about 5 minutes.
Stir in chicken mixture and reduce heat to medium-low. Simmer, stirring frequently, until chicken is no longer pink, about 10 minutes. Off heat, stir in remaining lemon juice and parsley.
For Toasted Bread: Adjust oven rack to middle position and heat oven to 400 degrees. Arrange eight 1-inch-thick slices of Italian bread on baking sheet. Brush 2 tablespoons of melted butter over both sides of bread slices.
Toast bread until golden brown, about 10 minutes, flipping slices halfway through cooking. Spoon chicken à la king over toast and serve.