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.
Notes
Make the crostini and chicken in advance for an even faster crostini!You may need more than one cup of chicken depending on how finely it is shredded, how big the crostini pieces are, and how much you want on each one!