In a large nonstick skillet over medium-high heat, add olive oil, mushrooms and onion; sauté for 8 minutes. Add vinegar to skillet, swirling a few times until vinegar becomes thick and syrupy, about 4 minutes. Add spinach and all spice, stirring until spinach wilts. Hold over low heat.
Lightly brush both sides of bread slices with olive oil, arrange in a single layer and lightly toast under broiler, about 1 to 2 minutes. Flip all slices and return to broiler to lightly toast the other side, about 1 to 2 minutes.
To serve, move spinach mixture from skillet to a medium bowl with a slotted spoon, leaving any excess juices in skillet. Add cheese to spinach mixture, stirring to incorporate. Top bread slices and serve.