Pre-heat the oven to 375ºF. Cut pears in half lengthwise and scoop the middle out with a melon baller or a spoon. Place them on a baking sheet lined with parchment paper or a silicon mat, cut side up and roast them for 10-20 minutes, depending on how firm and ripe the pears are.
Place the prosciutto slices on top of the pears. Arrange them in a way that they will cover the surface of the pear, but will leave the middle free. In the middle, scoop the teaspoon of gorgonzola cheese. Return baking sheet to the oven and roast for 5 more minutes.
Remove pears from the oven and let them rest to cool slightly.
Meanwhile, start making the dressing and plating the salad.
For the dressing, whisk all ingredients except for the olive oil in a bowl. Start adding the olive oil in a fine stream while whisking. This process should take a few minutes to complete, as you should slowly add the oil, to emulsify the dressing.
To plate, arrange 2 cups of arugula in each plate. Sprinkle ¼ cup of nuts over leaves. Place two pears on the top and drizzle dressing over.