Preheat oven to 350 degrees. Spray a baking sheet with cooking spray. Optional line with parchment paper first, then spray. Chop green onions and mushrooms very finely, to a nearly paste-like consistency. Melt butter in a small saucepan over medium heat; stir in green onions and mushrooms and cook and stir until fragrant, about 2 minutes. Whisk cornstarch into half-and-half; pour into mushroom mixture and cook, stirring often, until thickened, about 3 minutes. Season mushroom mixture with sage, salt, pepper, and garlic powder.
Roll bread slices with a rolling pin until very flat and thin. Spread mushroom mixture evenly onto flattened bread; tightly roll up bread slices. Slice each bread into 1-inch thick rounds; place rounds onto a baking sheet. Bake in the preheated oven until bread is browned, about 10 minutes.