Melt 1 tablespoon butter in a large nonstick skillet over medium-high heat. Add mushrooms, and sauté 10 minutes or until browned. Remove from heat.
Melt 3 tablespoons butter in a medium saucepan over medium heat; add onion, and sauté 5 to 7 minutes or until lightly browned. Whisk in flour. Cook, whisking constantly, until lightly browned (about 1 minute). Gradually whisk in broth and next 2 ingredients. Cook, whisking constantly, until thickened and bubbly. Stir in mushrooms, and cook, stirring constantly, until thoroughly heated. Garnish, if desired.