Heat the olive oil over medium heat in a Dutch oven or stockpot. Add the diced onions and stir, cooking until translucent.

Stir in the mushrooms, garlic, and red pepper flakes; cook until fragrant (about 1 minute).

Add the broth and salt (if using). Bring to a simmer, then stir in the beans. Remove ½ cup of the beans and broth.

Gently stir in the spinach until it is wilted and the beans are hot.

Just before serving, seed, peel, and dice the avocado. In a small blender, combine about ¼ of the avocado and the reserved beans and broth. Blend until smooth.

Stir the mixture back into the pot to thicken the stew. If a thicker stew is desired, continue simmering for a few minutes.

Taste and adjust seasonings if needed. Ladle the stew into serving bowls and top with the remaining diced avocado.