Heat the olive oil in a large pot or Dutch oven over medium heat. Add the onion and cook for 5 minutes, or until the onion is clear and soft, stirring frequently. Add the garlic and mushrooms, if using. Cook until the mushrooms have released their juices and reduced in size (4-5 minutes).
Add the lentils, quinoa, salt, thyme, and broth. Bring the mixture to a boil, then reduce heat to low. Cover and simmer for 30 minutes, or until the lentils are tender. If all of the liquid has been absorbed when you uncover the pot, add a splash of extra broth or water.
Add the tomatoes and baby spinach to the pot. Stir everything until the spinach has wilted completely into the quinoa and lentil mixture. Add a splash of vinegar or lemon juice, along with black pepper to taste and extra salt as needed. Finally, stir in the cashew cream, if using.
Leftovers will keep in an airtight container for up to 3 days.