Preheat the oven to 350° F.
Arrange baguette slices in an even layer on a baking sheet. Bake 1-2 minutes, just until lightly toasted. Allow the bread to cool then spread ricotta, to taste, on each piece.
In a medium skillet, heat olive oil over medium-high heat. Add the mushrooms and sauté for 3-5 minutes. Add garlic and continue cooking another minute or so.
Add wine and balsamic vinegar. Stir while liquids are reduced slightly. Stir in scallions and remove from heat.
Top each crostini with mushroom mixture, sprinkle with scallions, and serve.