Grill the bread, giving it a good searing on the edges and place on a tray.
Preheat a skillet and in it place the onions with a little olive oil. Sweat until translucent, and add the chopped prosciutto and the rosemary. Sauté until crisp, add the tomato paste and stir well.
Add the beans and the water, and season with salt and pepper to your liking. Cook until creamy looking and dry, remove from the fire and allow to cool well.
Once the beans are cooled to room temperature, add the shredded Grana Padano cheese and a little olive oil to bring it together. Mix well.
Spoon a mound of the beans on each of the crostini and top with a rosette of sliced prosciutto di parma.