Preheat your oven to 350° and lightly oil a 9x13-inch casserole dish.
Place the cauliflower in a large pot and fill it with enough water to cover it plus two inches. Bring the water to a boil over medium-high heat. Reduce the heat to low and allow it to simmer for about 10 minutes, or until the cauliflower is fork-tender
Drain the cauliflower, saving the cooking the water. Allow the cauliflower to cool slightly, and then place the it in a blender with 2 cups of the cooking water and the nutritional yeast. Blend it until it’s smooth and creamy.
While the cauliflower is cooking, place the garlic, sliced mushrooms, and oil in a large pan and cook over medium-high heat for about 10 minutes, or until the mushrooms are soft and the garlic is fragrant.
In a large bowl, mix together the cooked spaghetti, mushrooms and garlic, cauliflower cream, chickpeas, and peas, and season with salt and pepper to taste.
Place the mixture into the prepared casserole dish for 35-40 minutes, or until the mixture is bubbly and heated throughout.