Pour 1 ½ cups broth into the base of a 7-quart slow cooker. Add the chicken, onions, garlic, 2 teaspoons salt, pepper, and Italian seasoning.
Cover and cook on low for 1 hour.
Meanwhile, in a large pot, bring 2 to 2 1/2 quarts of water to a boil; just enough to cover the pasta (see note below). Cook the pasta for about 7 minutes, until al dente. It should still have a slight bite, as it will continue to soften slightly in the slow cooker.
Reserve ½ cup of the pasta water, then drain the pasta. Set aside.
Cut the Boursin into quarters and add it to the slow cooker. Stir until combined. Add the tomatoes and reserved pasta water. Cover and continue cooking for 30 minutes, until the tomatoes have softened.
Add the pasta and toss to coat. Season to taste with salt and pepper.
Cover and let sit for about 10 minutes before serving. The sauce will thicken and become creamy.
Give the pasta a final stir, then serve. Garnish with parsley if desired.