To make the hummus, place all ingredients in a food processor and blend until smooth. Adjust the amount of water and harissa pasta depending on how thick and spicy you want your hummus. It should still be spreadable. Set the hummus aside.

Slice the mushrooms and fry in a shallow pan until they are golden and crispy. Once they are ready, push them to one side of the pan and add the spinach to wilt.

To assemble the crostini, spread some hummus on a slice of the bread (you can toast it if you like), then top with some spinach and mushrooms. Serve immediately.