Cut baguette into 4 (1-inch-thick) slices and 8 (1/4-inch-thick) slices, and place on a baking sheet. Reserve remaining baguette for another use.
Bake bread at 400° for 5 minutes or until toasted. Set aside.
Melt butter in a large skillet over medium heat; add onion, and cook, stirring often, 20 minutes or until caramel colored. Add mushrooms and garlic, and cook, stirring constantly, 3 minutes. Stir in beef broth and wine; bring to a boil. Reduce heat, and simmer, stirring occasionally, 20 minutes or until slightly thickened.
Stir together 1/2 cup water and flour until smooth. Stir flour mixture and pepper into mushroom mixture; cook, stirring constantly, 1 minute.
Place thick baguette slices in bottoms of 4 (8-ounce) custard cups or small ovenproof bowls. Spoon mushroom mixture evenly into cups. Top each with 2 baguette slices. Sprinkle evenly with cheese.
Bake at 350° for 5 minutes or until cheese melts, and sprinkle with parsley.