Toast walnuts: While spaghetti squash is roasting, add 1 cup walnuts to a pan and place in the oven at 400°F for 10 minutes to toast. If you're using the roasted ring method for the squash you can do this at the same time. Remove walnuts from the pan to cool.
Sauté: In a large skillet, add 1 teaspoon olive oil over medium heat, add garlic and onion and sauté until fragrant and soft.
Make sauce: Transfer onions and garlic and walnuts into a blender and add water, nutritional yeast, dried parsley, salt and pepper. Blend until fully combined and smooth.
Toast walnuts: In the same large skillet, add remaining 1/4 cup walnuts to toast. Add additional oil, if needed. Once toasted, set aside.
Combine and toss: Add cooked spaghetti squash noodles, peas and the walnut sauce into the same large skillet you used for toasting the walnuts and toss to combine. Stir in most of the chopped fresh parsley, reserving a bit for garnish.
Serve: Plate spaghetti squash pasta and top with crushed red pepper, toasted walnuts and fresh parsley. Serve warm.