Prepare the Whole-Grain Foundation. If your bread is fresh, preheat your oven to 175°C (350°F). Scatter the torn whole-wheat sourdough pieces on a baking sheet. Bake for 15-20 minutes, or until the bread is completely dry and crisp. This step is crucial for ensuring the bread thickens the soup without turning mushy.
Bloom the Aromatic Base. In the Dutch oven, heat 3 tablespoons of extra-virgin olive oil over medium heat. Add the onion, leek, and carrots. Cook, stirring occasionally, for 12-15 minutes until the vegetables are exceptionally soft and sweet. Do not rush this stage; it builds the primary layer of flavor.
Infuse the Herbs and Spices. Add the minced garlic, chopped rosemary, thyme, and optional red pepper flakes to the pot. Stir constantly for one minute until fragrant. Pour in the white wine to deglaze the pot, scraping up any browned bits from the bottom. Allow the wine to reduce by about half.
Build the Savory Broth. Stir in the undrained diced tomatoes, low-sodium vegetable broth, rinsed cannellini beans, the Parmesan rind, and the bay leaf. Increase the heat to bring the mixture to a gentle boil, then reduce to a low simmer.
Simmer and Meld. Cover the pot and let the soup simmer for 25 minutes. This period allows the Parmesan rind and bay leaf to infuse the broth with deep, savory umami notes and melds the flavors of the vegetable base.
Wilt the Greens. Uncover the pot. Remove and discard the Parmesan rind and bay leaf. Stir in the chopped kale and cook for 5 minutes, until the leaves are tender and have turned a vibrant dark green.
Thicken and Finish. Turn off the heat. Stir in half of the toasted whole-wheat sourdough pieces until they begin to break down and thicken the soup. Season generously with freshly ground black pepper and stir in the fresh lemon juice to brighten the flavors.
Serve and Garnish. Ladle the Ribollita into bowls. Top each serving with the remaining toasted bread pieces, a scant teaspoon of grated Parmesan cheese, and a final, generous drizzle of high-quality extra-virgin olive oil.