Prep step: Defrost Greens. Defrost your frozen spinach in the microwave, according to the package instructions. Allow to cool, then wring out most of the excess in the spinach moisture with a clean dishtowel. What little moisture remains can cook-off on the stove, so do not feel like you need to strive for perfectly dry spinach.
In a large pot over medium-low heat, warm the olive oil. Once warm, add the onions and sauté until onion is translucent.
Once onions begin to become translucent, add garlic and sauté until fragrant, around one to two additional minutes. Stir consistently, being careful not to let the garlic burn.
Season onions and garlic with salt and pepper, to taste
Add defrosted spinach to the pot and cook for an additional 2-3 minutes, allowing additional moisture from the frozen spinach to evaporate.
Add vegetable broth, drained and rinsed cannellini beans, and nutritional yeast.
Bring the soup to a boil, then lower the heat, allowing the mixture to simmer for twelve minutes.
Adjust seasoning as needed and remove soup from the heat.
Serve warm, with a pinch of red pepper flakes and an extra drizzle of olive oil, if desired.