Boil a large, salted pot of water for the casarecce pasta. Cook it al dente (8 minutes).
Meanwhile, prep your vegetables. You can do some prep while the sausages are cooking to save time, but feel free to prep everything prior to starting the sauce if that's easier for you.
Take the sausage meat out of the casings and crumble it into a deep skillet. Cook it over medium-high heat, stirring occasionally, until it's browned (about 5-7 minutes). Transfer to a plate.
Leave the sausage grease in the pan and then add the mushrooms and onions (if there is very little grease then add about a teaspoon of olive oil). Sauté, stirring occasionally, until the mushrooms release their water and they get a nice sear and the onions brown (about 5-6 minutes).
Stir in the garlic and cook for 30 seconds.
Add the tomatoes, peppers, chicken broth, and sausage to the skillet. Reduce the heat to medium and cook for around 5 minutes or until the tomatoes have started to lose their shape and the chicken broth has reduced a bit.
Add in the pesto (stir the jar first), then take the pan off the heat and ensure it's stirred in well. Add a splash of pasta water to the sauce (a couple tablespoons) prior to draining it, and then toss the drained pasta with the sauce. Season with salt & pepper as needed. Serve immediately with parmesan cheese and parsley if desired.