This recipe has a comforting lasagna vibe, as the spicy chicken-and-tomato sauce mingles with the rich, herby ricotta. Make the sauce as hot as you like: Begin with 1 1/2 teaspoons red-pepper flakes for a fairly mild spice level, then add more at the end if you want more heat. You’ll notice there isn’t much liquid when the chicken and vegetables go into the slow cooker, but have faith: The juices released during cooking end up making a not-too-thick ragù that is perfect for saucing pasta. In fact, because some chicken has more moisture than others, you may end up with more liquid than you’d like. If that happens, turn off the heat so the sauce cools a bit and is not bubbling, then vigorously stir in two egg yolks before you add the pasta. This adds glossy thickening power, just like it does in carbonara.