Preheat the oven to 400°F (200°C).

Drain and rinse the white beans; set them aside.

Heat 1 tablespoon olive oil in a large stock pot over medium heat. Add the diced sweet onion and a big pinch each of kosher salt and pepper. Cook, stirring occasionally, until the onion is softened, about 5 minutes.

Add the minced garlic, ½ teaspoon dried thyme, and ½ teaspoon dried rosemary to the pot. Cook, stirring, for about 30–60 seconds until fragrant.

Add 5 to 6 cups cauliflower florets, the drained and rinsed white beans, and 4 cups vegetable or chicken stock. Add another pinch of salt and pepper. Bring to a boil, then reduce the heat to a simmer. Cover and simmer for 15–20 minutes, until the cauliflower is very tender.

While the soup simmers, arrange 3 to 4 cups sourdough bread chunks in a single layer on a baking sheet.

In a small bowl, whisk together 3 tablespoons melted butter and 2 tablespoons Dijon mustard. Drizzle the mixture over the bread chunks and toss until all pieces are evenly coated. Sprinkle with salt and pepper.

Roast the bread in the preheated oven for 10–12 minutes, until golden and toasty. Remove from the oven and set aside.

When the cauliflower is tender, remove the pot from the heat and let it sit for a couple of minutes to cool slightly. Working in batches, carefully transfer the soup to a high-powered blender and puree until completely smooth (hold the lid and vent slightly or use a towel over the lid to avoid hot splashes). Return the pureed soup to the pot.

Taste the soup and season with additional kosher salt and pepper as needed. Keep the soup over low heat until ready to serve.

Ladle the soup into bowls, garnish with fresh thyme leaves, and top with the mustard croutons. Serve immediately.