Heat the oven to 400 degrees or prepare a barbecue.
Use half of the oil to brush each slice of bread.
Cook in the oven for 10 minutes or until the grill marks appear.
Meanwhile, put the remaining oil in a pan over medium heat, add the garlic and onion, cook until translucent.
Add the beans and spinach, cook for about 5 minutes until they wilt.
Add the sage, cover and cook.
If the mixture seems too dry, add a little water.
Remove from heat, season with salt and pepper.
Spoon over the crostini, sprinkle with parmesan and serve immediately.