Set a fine mesh strainer over a medium bowl and add the beans, catching the liquid in the bowl. Add ½ up of beans to the bowl and set aside the remaining beans in a small bow. With a fork, mash the beans into liquid to a creamy consistency.
In a large skillet sauté the oil, onions, garlic, pancetta and rosemary for 3 minutes on medium-low heat.
Stir in tomatoes, add the salt and the crushed beans with the strained liquid to skillet and simmer for 15 minutes on low, stirring frequently.
Remove sprig of rosemary and add the reserved whole beans to skillet and simmer 10 more minutes, stirring frequently.
Bring 3 quarts of water to a boil, add 1 tablespoon of salt (or amount desired), add tagliatelle to water and cook for 10 minutes.
Drain the pasta, add to skillet and stir-fry for 30 seconds until pasta has absorbed the sauce.
Serve with grated Parmigiano Reggiano cheese.
For an equally delicious vegetarian version, omit the pancetta in step 2.