In a large skillet over medium heat, add oil. Add onion and saute till translucent, approximately 5 minutes.
Add milk, peas, and prosciutto and bring to a simmer; cook 3 to 4 minutes till peas are warmed through.
Add zest, juice, salt, and pepper and mix together.
Add cooked pasta and grated parmesan to the skillet and toss with pea mixture. Add small amounts of reserved pasta water, tossing all ingredients together and heating through, till desired sauce consistency reached.