Preheat your oven to 400°F.
Peel and cube the butternut squash into 1-inch pieces. Toss with a small amount of olive oil, salt, and pepper, and spread on a baking sheet.
Roast the squash in the oven for 25-30 minutes until tender and slightly caramelized. Meanwhile, dice the onion and mince the garlic.
Heat a small pan over medium heat with a drizzle of olive oil. Sauté the onion and garlic until soft and fragrant, about 5 minutes.
In a large pot, combine the roasted squash, sautéed onion and garlic, vegetable broth, red lentils, and cannellini beans. Bring the mixture to a gentle simmer.
Allow the soup to simmer for 10-15 minutes so the flavors meld, then remove from heat.
Using an immersion blender (or a countertop blender in batches), blend the soup until smooth and creamy. Adjust seasoning with salt and pepper as needed.
In a small pan, heat a tiny drizzle of olive oil over medium heat and quickly fry the sage leaves until crisp, about 1 minute. Remove and set aside on a paper towel.
Ladle the soup into bowls, and top each serving with a dollop of plain nonfat Greek yogurt and a few crispy sage leaves for garnish.
Serve warm and enjoy your comforting, protein-enhanced butternut squash soup.