1) Preheat the oven to 180°C/gas mark 4.
2) Place the prosciutto slices on a baking sheet and bake until just crisp, about 10 mins. Let it cool. Crumble the prosciutto slices into a container and store in the refrigerator.
3) In a blender, combine the orange zest, orange juice, balsamic vinegar, honey, garlic, salt, and pepper. Blend until smooth. With the blender running, add the olive oil in a steady stream until combined. Transfer to a container and store in the refrigerator.
4) To serve, put the spinach in a large bowl. Toss with enough of the vinaigrette to coat the spinach. Sprinkle with the crisp prosciutto crumbles, toss again, and serve.