Add garlic to the pot, saute and stir for a minute more, and then add the cubed squash, rosemary, salt, and lentils. Mix well.
Add the broth and allow the pot to simmer for about 1 hour, until both the lentils and squash soften.
Once the soup is cooked to your liking, place about 4 cups of soup in a blender along with 1/2 cup of olive oil. Blend until the emulsion is creamy, then add the mixture back to the soup pot.
Off the heat, add the spinach, parsley, and sherry vinegar. Add salt and pepper to taste.
Top each serving with freshly grated Parmesan cheese and a little extra vinegar or a squeeze of fresh lemon juice, if desired.