Cook pasta as directed. Drain and set aside. Reserve ½ cup of the pasta water.
Heat olive oil in a large heavy bottom skillet over medium heat, add prosciutto. Cook till prosciutto has browned, about 3 to 5 minutes. Add minced garlic. Cook 30 seconds.
Mix in flour. Cook for 2 to 3 minutes, stiring constantly.
Stir in milk a half a cup at a time. Then add frozen peas to the skillet. Stir every 30 seconds until it starts to thicken, about 10 min. You'll know its done when the sauce thickly coats the back of a spoon. .
Remove sauce from heat and stir in mozzarella. Salt and pepper to taste. If the sauce is a little thick add in pasta water a teaspoon at a time till it is the desired consistency.