Step 1

In a large pot over medium-high heat, melt 2 tablespoons butter with 2 tablespoons oil

Step 2

Set aside 1 ½ cups torn bread for thickening the soup

Step 3

Add the remaining bread, ½ teaspoon thyme, ¼ teaspoon each salt and black pepper, and the pepper flakes (if using) to the pot

Step 4

Cook, stirring occasionally and lowering the heat as needed to prevent the seasonings from burning, until the bread is golden brown, 2 to 4 minutes

Step 5

Transfer to a large plate, toss with the Parmesan and spread in a single layer

Step 6

Set aside to cool and crisp

Step 7

In the same pot over medium, melt the remaining 2 tablespoons butter

Step 8

Add the onion, garlic, ½ teaspoon salt and the remaining ¼ teaspoon thyme; cook, stirring occasionally, until the onion is translucent but not browned, 5 to 7 minutes

Step 9

Stir in the tomatoes, the 1 ½ cups reserved bread and 4 cups water

Step 10

Cook, uncovered and stirring occasionally, until the bread is completely soft and the broth is slightly thickened, about 10 minutes

Step 11

Remove from the heat and cool for about 5 minutes

Step 12

Using a blender and working in batches so the jar is never more than half full, puree the tomato mixture, streaming in the remaining 3 tablespoons oil, until smooth

Step 13

Return the soup to the pot

Step 14

(Alternatively, if you own an immersion blender, puree the soup directly in the pot; when almost smooth, slowly stream in the remaining 3 tablespoons oil while blending

Step 15

) Cook over medium-low, stirring often, until heated through, 2 to 5 minutes

Step 16

Thin the soup with water, if needed, to reach the desired consistency

Step 17

Off heat, taste and season with salt and black pepper

Step 18

Serve topped with the croutons and basil pesto (if using) and sprinkled with additional Parmesan