Start the soup: Heat the olive oil in a large skillet over medium-high heat. When it is hot, add the onions, 1 teaspoon of the salt, and the crushed red pepper. Cook, stirring occasionally, until the onions have softened, about 3 minutes. Add the sliced garlic and continue to cook, stirring, for 3 minutes longer.

Transfer the sautéed vegetables to a 6-quart slow cooker and add the tomatoes, chicken stock, thyme and basil sprigs, the remaining 1 tablespoon salt, and the Parmesan rind.

Cook on high for 6 hours, stirring occasionally.

While the soup is cooking, make the croustades: Preheat the oven to 400°F.

Place the torn bread pieces on a large baking sheet and drizzle with the olive oil. Toss with your hands to evenly distribute the oil. Season lightly with salt and pepper, and bake until golden brown and crisp, 12 to 15 minutes. Set aside to cool.

To make the basil oil, combine the olive oil, basil leaves, and minced garlic in a blender or processor. Process in short bursts until the leaves are finely chopped—don’t over-process or the leaves will lose their bright green color. Transfer to a small bowl and season to taste with salt.

When you are ready to serve the soup, remove and discard the Parmesan rind and thyme sprigs. The basil sprigs should be very soft and can be left in or discarded, as preferred. Taste the soup, and adjust the seasoning if necessary. Ladle the soup into wide shallow bowls and garnish with some of the croustades and Parmesan shavings. Drizzle the basil oil over the soup to taste, and then sprinkle with pepper.