Heat olive oil in a large pot over medium heat. Add diced onion and sauté until translucent, about 5 minutes. You'll notice a sweet aroma as the onions cook.

Stir in minced garlic and cook for an additional minute until fragrant, being careful not to brown the garlic.

Add cannellini beans and vegetable broth to the pot. Bring to a gentle simmer, scraping any fond from the bottom of the pot.

Toss in the Parmesan rind and fresh rosemary. Let the soup simmer for 20 minutes, allowing flavors to meld. The aroma will become more complex.

Remove the Parmesan rind. Use an immersion blender to blend part of the soup for a creamy texture, leaving some beans whole for contrast.

Season with salt, pepper, and red pepper flakes if using. Taste and adjust seasoning as needed.

Serve the soup hot, drizzled with a little olive oil for richness and a sprinkle of extra rosemary if desired.

For added texture, consider serving with crusty bread or croutons on the side.