As with many soups, this one is about the add-ons. Serving it with the three suggested toppings (croutons, pesto, scallions) will take this to the next level, but you’re still OK if you only have the time or sanity for the first two. Tip for parents serving soup skeptics: The kids might think of the soup as more of a “dip” if you serve it in a small bowl and the crouton like a piece of toast on the side.
- In a stockpot set over low heat, combine 3 tablespoons of the olive oil, the onion, salt, black pepper, and red pepper flakes. Sauté until the onion is golden and slightly caramelized, stirring often, for as long as you have patience, but a minimum of 5 to 7 minutes. (A lot of the depth in this soup will come from caramelizing the onion, so the longer, the better.) Add the garlic and cook until fragrant, about 2 minutes. Add the beans and just enough stock to cover. Increase the heat to medium-high and bring the mixture to an aggressive simmer until warmed through, about 3 minutes.
- Remove the pot from the heat and, using a slotted spoon, scoop out and set aside about 1 cup of the beans. Stir the vinegar into the pot, then puree the soup using a handheld immersion blender. Set back on the stove top over low heat to keep warm.
- Preheat the broiler. Place the bread slices on a sheet pan and broil on the top rack of the oven just until toasted but not quite golden, about 1½ minutes. (Watch carefully so they don’t burn.) Flip them over, brush with the remaining 2 tablespoons olive oil, sprinkle each with 1 tablespoon Parmesan, and broil until the cheese has melted, another 1½ minutes. Remove from the oven.
- Serve the soup in large shallow bowls, placing one giant crouton in the middle of the soup along with a few reserved beans, the pesto, and the scallions and chives, if using. Drizzle with more olive oil just before serving.
- Toss one bunch trimmed scallions, with stems, in 2 tablespoons olive oil, place on a foil-lined sheet pan, and broil for 5 minutes; let cool and roughly chop)
- Make an ice bath by filling a medium bowl with ice and water. Bring a large pot of salted water to a boil and add the kale. Cook until wilted and softened, about 2 1/2 minutes. Reserve 1/3 cup of the cooking water and, using tongs or a slotted spoon, transfer the kale to the ice bath to stop the cooking process and preserve its bright green color. Drain and cool until the leaves are comfortable to handle. Place the leaves between two paper towels and wring out as much moisture as you can.