Preheat the oven to 400 degrees.
Trim the stems and greens off the beet. Wrap the beet with parchment paper or foil on a sheet pan and roast for 50-60 minutes or until the beets are tender. You can test it by sticking the beet with a fork or knife, it should slide in easily. It may take less time if you use small beets.
Put the raw cashews in a medium saucepan and cover with 1-inch of water. Bring it to a boil and let it cook for 10 minutes. Drain any excess water. Set aside.
While the beets are roasting, saute the onions in 3-4 tbsp of vegetable broth in a large pan over medium heat until they're soft and translucent. Add the sliced garlic and let it cook for 3 minutes.
Remove the cooked beets from the oven and let them cool until you can handle them without burning yourself. You can cut the skin off or use a paper towel to rub the skin off.
In a large pot, boil water and cook the pasta until tender. Drain the pasta and set it aside. While the pasta is cooking, make the beet sauce.
Add the cooked and peeled beet, boiled cashews, sauteed onions and garlic, 3 tbsp lemon juice, 1 cup almond milk, ½ tsp salt, and ¼ tsp black pepper to a high-powered blender. Blend on high until the sauce is smooth and creamy. Pour the sauce into the pan that you cooked the onions in and warm it over medium-low heat.
Add the zest of 1 lemon, 3-4 handfuls of fresh spinach, and the cooked noodles to the pan with the beet sauce. Stir to coat the noodles in the sauce. Taste and add more salt and pepper if necessary. Garnish with chopped chives before serving.