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1
Heat the olive oil in a large saucepan over medium heat until shimmering. Add the onions and red pepper flakes and cook, stirring occasionally, until the onions are translucent and very tender, about 10 minutes.
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2
Stir in the tomatoes and their juices, plus the water, and bring to a boil. Reduce the heat to low and simmer, stirring occasionally, until the flavors have melded, about 30 minutes. (If you're in a hurry, you can skip the simmer time -- just add a bit less water.)
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3
Add the basil, season with salt and pepper, remove from the heat, and let cool briefly, about 5 minutes.
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4
Set a fine-mesh strainer over a large, heatproof bowl.
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5
Using a blender, puree the soup in batches until smooth, removing the small cap from the blender lid (the pour lid) and covering the space with a kitchen towel (this allows steam from the hot soup to escape and prevents the blender lid from popping off).
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6
Pour the blended soup through the strainer, pressing on the solids with a rubber spatula or ladle; discard the solids. Taste the soup and season with additional salt and pepper as needed.
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7
Return the soup to the saucepan and reheat on medium low until hot.
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8
If you choose, serve topped with a tablespoon of creme fraiche.
