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.

Add olive oil to a large skillet over medium-high heat.  Add the mushrooms and sauté for 3 minutes, until softened. Add garlic and continue cooking for an additional 1 minute.

Add white wine, balsamic vinegar, salt, and pepper. Cook, stirring frequently for 1 minute; liquid will be reduced slightly. Remove from heat and stir in 1/2 of the scallions.

Top each crostini with a heaping spoonful of the mushroom mixture, garnish with a sprinkling of the remaining scallions, and serve warm.