Add 1/2 cup cashews (60 g // adjust amount if altering number of servings) to a bowl and cover with warm water by at least 2 inches. Set aside to soak.
Clean the mushrooms by wiping them with a clean, damp kitchen towel. Thinly slice 1 ½ cups (105 g) of the mushrooms and add the remainder to a food processor with the “S” blade. Pulse to finely chop into small pieces. If you do not have a food processor, you can finely chop with a knife. Set aside.
NOODLES: Add 3 quarts (~3 liters) of water and 1 Tbsp salt to a large pot (adjust amounts if adjusting serving size). Bring to a boil. Once boiling, add pasta, stir, and set a timer for the lower end of your package’s recommendation.
Meanwhile, drain the cashews and add them to a small blender with 1 cup (240 ml) water (adjust amount if altering batch size). Blend until smooth to make cashew cream. Set aside.
Test pasta for doneness. When it’s ready, reserve 1/2 cup (120 ml) pasta water for thinning the sauce. Then drain the pasta and set aside.
STROGANOFF: Melt the vegan butter in a Dutch oven or large cast iron skillet over medium heat. Once melted, add the onion and season with a healthy pinch of salt and pepper (optional). Cook for 3-4 minutes or until translucent. Add the sliced mushrooms ONLY (not the finely chopped) and cook for 8-10 minutes, stirring occasionally, until they have released most of their moisture and begin to brown.
Add the remaining (finely chopped) mushrooms and cook for 5-7 minutes. Add the garlic and thyme and sauté until fragrant — about 1-2 minutes. Add the cornstarch (or flour), salt, pepper (optional), and mustard and stir to coat the vegetables in the cornstarch (or flour).
Add the cashew cream and bring to a simmer. Cook until thickened but still saucy, stirring frequently, over medium-low heat. If you want it saucier, you can stir in the 1/4 - 1/2 cup (120 ml) reserved pasta water. Or, if you forgot to reserve pasta water, you can also use filtered or tap water if needed. Serve over noodles or add the noodles to the sauce and stir to combine.
Serve warm and optionally garnish with chives and vegan sour cream or yogurt. Leftovers will keep stored separately for 2-3 days. Sauce can be stored in the freezer for up to 1 month. Reheat leftovers in a saucepan until warm, adding water as needed to thin.
*If cashew-free, you can use 1 cup light coconut milk and 1/2 cup water in place of the 1 ½ cups cashew cream. *Nutrition information is a rough estimate calculated with gluten-free brown rice and quinoa fusili, 1/4 tsp salt in the pasta water (since the full tablespoon isn’t all absorbed into the pasta), and without optional ingredients.