• 1 1/2 c. cooked cannellini beans, with some of the liquid
Bring a large pot of water to boil and salt it. Put oil in large skillet over medium heat. When it's hot, add mushrooms, sprinkle with salt and pepper, and raise heat to medium-high. Cook, stirring occasionally, until they release their liquid and the pan begins to dry, 8 to 10 minutes.
Add leek and garlic, and sprinkle with salt and pepper. Cook, stirring often, until soft, 3 to 5 minutes. Add kale and paprika, toss to coat, then pour in wine and cook, scraping any browned bits off the bottom of the pan, until the wine has reduced by half, just a minute or 2.
Add stock and mustard and bring to a boil. Adjust heat so mixture bubbles gently and cook, stirring occasionally, until kale stems are soft and leaves are silky, 10 to 15 minutes.
Meanwhile, add pasta to boiling water, and cook, stirring occasionally, for 5 minutes, then start tasting. When pasta is tender but still has some bite, scoop out 1 cup cooking water, then drain.