Drain the water from the soaked cashews. Blend the drained soaked cashews with 1 cup of water in a powerful blender if possible (such as a Vitamix) until completely creamy. A regular blender will work as well, just be sure to have soaked the cashews for about 4 hours prior to blending to get that creamy consistency.
To Make the Pasta:
Preheat the oven to 400 degrees.
In a large bowl, toss the quartered mushrooms with a tiny amount of olive oil, salt, and freshly ground black pepper.
Line a baking sheet with parchment paper, spread the mushrooms in a single layer on the prepared baking sheet and bake in the oven for 15-20 minutes until the mushrooms are roasted. Once done, set aside.
Boil a large pot of water, add some salt and olive oil, and boil the pasta a la dente (be sure not to overcook this pasta shape otherwise it will loose it's form). Once done, drain and set aside. If you plan on cooking the pasta ahead of time, rinse with cold water to stop the pasta from cooking and set aside. While pasta is cooking, continue on to prepare the sauce.
In a large non-stick pan, heat 3/4 cup vegetable broth. Add 1/4 cup + 2 tbsp of the prepared cashew cream and slowly mix in completely with the vegetable broth (you can freeze the leftover cashew cream for use later).
Add the pesto, completely combine, and let it heat on medium-low slowly, mixing frequently, until the sauce thickens.
Add in the cooked rigatoni pasta and roasted mushrooms. Toss with the creamy pesto sauce, season with salt and freshly ground black pepper, and let the pasta/mushrooms heat through if needed. Serve hot!