Step 1Cook pasta in plenty of rapidly boiling salted water until almost tender, about 10 minutes. Drain and set aside, reserving some cooking water.
Step 2Combine prosciutto, butter, lima beans and 2 tablespoons reserved pasta cooking water in cold skillet and cook, covered, over medium heat, stirring occasionally, until beans begin to soften, about 5 minutes.
Step 3Add squash, replace cover and cook, stirring occasionally, until squash softens and begins to glaze, about 5 minutes more.
Step 4Add tomatoes and cooked pasta to skillet and cook, stirring, until tomatoes begin to melt into sauce, about 3 minutes. Season to taste with salt and pepper (salt lightly at first; because pasta cooking water is salted, you might not need to add much). Dust with most of grated cheese and divide among 6 heated pasta bowls. Garnish with remaining cheese and serve.