Prepare the chard by trimming the stem ends and discarding them.Then trim the leaves from the large center stems. Set the stems aside and pile up the leaves. Slice the stems into 1/2-inch pieces. Pile 4-5 leaves on top of each other, roll tightly, then slice into 1 inch ribbons. Repeat with the remaining leaves. Cut the strips of leaves again sideways so that they are roughly 1- by 3-inch rectangles.
Put a large pot of water on to boil over high heat. This will be for the pasta.
Heat 1 tablespoon of olive oil in a separate large deep, skillet or Dutch oven over medium-high heat. Add the sausage in chunks and flatten each piece with the back of a wooden spatula or spoon. Flip the sausage pieces occasionally until they are golden brown, then remove them to a plate lined with paper towels. Drain the fat from the pan. Once the sausage has cooled a bit, break it up into smaller pieces with the wooden spatula or spoon you’ve been cooking with.
Return the Dutch oven to the stove. Lower the heat to medium and add 1 tablespoon of olive oil to the pan. Put in the chopped onions and sprinkle them with 1/4 teaspoon of salt. Cook for about 5 minutes, stirring frequently, then add the chard stems. Cook another 5 minutes, stirring frequently and scraping up any browned bits from the bottom of the pan. (I added a tablespoon or two of water when the onions and chard stems started sticking.)
Add garlic to the onion/chard stem mixture and cook for about 1 minute. Then add the chard leaves, the chile flakes, and about 1/4 teaspoon salt. Cook, using tongs to lift the chard leaves near the bottom of the pan to the top over and over so that they all cook evenly. After about 1 minute, add the broth to the pan. Lower the heat to simmer and cook about 8 minutes. Add the sausage and drained beans to the pot.
In the meantime, cook the pasta according to the package directions until it is almost al dente. (It will cook further with the chard mixture later on.) Scoop up about 1 cup of pasta water and put it aside before draining the pasta. Drain the pasta in a colander and add it to the chard mixture.
Stir the pasta into the warm chard, bean, and sausage mixture. Add 1/4 cup of pasta water along with the lemon zest and toss the pasta mixture over low heat for 2-3 minutes. Add more pasta water in 1/4 cup increments to keep the pasta loose, but not soupy. Stir in 1 tablespoon of butter and allow it to melt and coat the pasta mixture. Add half of the Parmesan cheese and the walnuts to the mixture, stir again, and dish out into servings bowls.
Offer more Parmesan for each person to add as they like.