Melt butter in a medium saucepan over medium-low heat. Add onions and mushrooms and sauté until golden brown, about 15 minutes. Add garlic powder and salt and pepper to taste. Add in flour and then, slowly, pour in milk. Continue to stir until thickened.
Flake tuna and stir in with a fork until thoroughly distributed. Remove from heat. Serve over toasted baguettes or slices of bruschetta.