Step 1: Heat the olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add the chopped onion and sauté for 5-7 minutes until softened and translucent.
Step 2: Add the minced garlic, chopped carrots, and celery. Cook for another 5 minutes, stirring occasionally, until the vegetables start to soften.
Step 3: Stir in the fresh sage leaves and rosemary sprig (if using). Cook for 1 minute until fragrant.
Step 4: Pour in the vegetable broth and crushed San Marzano tomatoes. Bring the mixture to a gentle simmer.
Step 5: Add the drained and rinsed cannellini beans. Season with salt, black pepper, and a pinch of red pepper flakes (if desired).
Step 6: Reduce the heat to low, cover the pot, and let the stew simmer for at least 30 minutes, or up to an hour, allowing the flavors to meld beautifully.
Step 7: Before serving, remove the rosemary sprig. Taste and adjust seasonings as needed. If the stew is too thick, add a little more broth or water.
Step 8: Ladle the hot stew into bowls. Drizzle with a good quality extra virgin olive oil and garnish with fresh parsley. Serve immediately with crusty bread.