Bring a large pot of salted water to a boil. Cook the rigatoni according to package directions until al dente. During the last 2 minutes, add the fresh English peas to the pot with the pasta. Before draining, reserve at least 1 cup of pasta water. Drain the pasta and peas together.
While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet over medium heat. Lay the prosciutto slices flat and cook until crispy, about 2–3 minutes per side. Remove to a paper towel–lined plate and let cool, then crumble or break into pieces. Leave the drippings in the skillet.
Reduce heat to medium-low. Add the remaining 2 tablespoons of olive oil and the sliced garlic to the skillet with the prosciutto drippings. Cook until the garlic is just golden and fragrant, about 1 minute. Don't let it burn.
Add the lemon juice and ¾ cup of reserved pasta water to the skillet. Stir and let it simmer for about 1 minute to come together into a light, silky sauce.
Add the drained pasta and peas to the skillet. Toss everything together over medium-low heat, adding the Parmesan and tossing until the sauce is glossy and coats the rigatoni. If it looks tight, add a splash more pasta water.
Season with salt, pepper, and red pepper flakes if you like.
Transfer to a serving bowl or individual plates. Top with generous dollops of ricotta, the crispy prosciutto, lemon zest, and a little more Parmesan.