Preheat oven to 350F.
Lay the baguette slices on a baking sheet either lined with parchment paper or on a silicone may. Drizzle with olive oil.
Bake the baguette slices until they are just starting to harden on top, approximately 5 minutes. Flip them over, drizzle lightly with olive oil, and bake until the second side is just starting to crisp up, approximately 3-5 more minutes.
Remove the baguette slices from the oven and set aside.
While the baguette slices are toasting: In a large nonstick skillet, melt the butter over medium heat. Add the mushrooms, sage, salt, and pepper and saute until the mushrooms are just cooked, approximately 5 minutes.
When you are ready to serve, place the crostini pieces on a serving platter or cutting board. Top each bread slice with 1-1.5 Tbsp of shredded chicken. Spoon some of the butter and mushrooms over each slice. Serve immediately.