Heat oil in a soup pot over medium heat. Add onions and garlic. Sauté until fragrant and onions are translucent.

Add sliced mushrooms to pot along with a pinch of salt. Cook for a few more minutes, stirring occasionally, until mushrooms release liquid and soften.

Add vegetable broth, brown and red lentils, barley, herbs de Provence, and 1/4 teaspoon salt. Bring to a medium-high heat until it reaches a low boil.

Turn heat to low, add chopped collard greens (optional). Stir and cover the pot. Cook for 25-30 minutes, or until barley is tender.