In a medium stockpot, add the beans and cover with water by about 2-inches above the surface of the beans. Cover the pot and bring to a boil. Lower the heat to medium-low and gently simmer until the beans are just tender. This should take about 45 minutes or so.

When the beans are done, drain them in a colander, reserving about 1 1/2 cups of the cooking liquid. Using a hand blender or food processor, purée the beans until smooth and creamy with just enough of the cooking liquid to thin the puree to the desired consistency, usually about 1 cup.

Put the flour, minced rosemary, salt, and pepper in a large mixing bowl and add about one-half of the boiling water. Using your hands, begin to incorporate the ingredients, adding more water, a little at a time, just until the ingredients come together to form a solid dough. Knead the dough until soft and smooth, for about 10 minutes. Cover the dough in plastic wrap and let rest for 30 minutes at room temperature.

Working on a lightly floured work surface, cut off a small piece of dough and roll it out with your hands into a long cylinder about 1/3-inch in diameter. Using a sharp knife, cut the cylinders into 1 1/3-inch pieces. Using your index and middle finger press on the “pillow” curling it towards you, letting the dough wrap over your finger and create a gap in the center. Or, you can place a knife on one end of the "pillow" and slide it toward you to do the same. Transfer the pieces to a large tray lined with a lightly floured tea towel as you work.

Bring a large pot of water to a rolling boil. Meanwhile, Heat a large sauté pan over medium-high heat. Add the mussels, sprig of rosemary, and a small ladleful of water and cover the pan. Sauté until the shells open, about 2 to 4 minutes, shaking the pan occasionally. Remove the pan from the heat.

Using a fine-mesh sieve, strain the mussels over a bowl to get rid of any sand and grit, reserving the juices. Discard the unopened shells. Remove the meat from the remaining shells and place in the bowl with the juices.

Drop the pasta into the boiling water (do not salt the water). Cook until the pasta is soft, but with a chewy bite, about 10 to 12 minutes, stirring occasionally.

Meanwhile, in a large sauté pan over medium heat, combine the olive oil and garlic and stir for 30 seconds. Add the mussels and their reserved juices and cook for 1 minute more. Using a spider strainer, transfer the pasta directly into the sauce and toss to combine. Add the tomatoes and cook for about 2 minutes, or until the pasta has absorbed some of the sauce, tossing occasionally.

Spoon the bean puree onto the center of each plate, creating a little bed. Portion the pasta and mussels over the top. Drizzle with a little olive oil and sprinkle with rosemary leaves. Serve hot.