If you have a panini press, use it to cook the grilled cheese croutons (it should take about five minutes). The grill marks add a nice touch. If you have a high-powered blender (like a Vitamix ), straining the soup isn't necessary; just blend until silky smooth.
2 tablespoons extra-virgin olive oil
1 small yellow onion, peeled, halved, and sliced thinly
1 teaspoon red pepper flakes
2 (28-ounce) cans whole tomatoes
1 1/2 cups water
1/4 cup loosely packed fresh basil leaves, plus more for garnish
About 2 teaspoons kosher salt, or to taste
About 1 tablespoon balsamic vinegar, or to taste
4 (1/2-inch-thick) slices rustic white bread
2 tablespoons unsalted butter, melted
4 ounces Gruyère cheese, grated
Neutral oil, such as grapeseed or canola
Freshly ground black pepper
- 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, are beginning to brown, and are very tender, about 10 minutes.
- Stir in the tomatoes and their juices, plus the water, and bring to a boil. Reduce the heat to low and simmer, stirring occasionally, about 30 minutes. Add the basil, salt, and balsamic vinegar, remove from the heat, and let cool briefly, about 5 minutes.
- Set a fine-mesh strainer over a large, heatproof bowl. Using a blender, puree the soup in batches until smooth. 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 to taste  with additional salt and vinegar as needed.
- Brush melted butter on one side of each slice of bread, making certain to cover the entire surface with butter. Turn the slices over, and pile Gruyère on two of the slices. Place the remaining two slices of bread on top of the Gruyère, buttered sides up.
- Heat a cast-iron skillet over medium-low heat, barely coat the pan with a drizzle of neutral oil, and cook the grilled cheeses for about 2 minutes per side, or until the cheese has melted and the bread is crusty and deep golden brown. For the best results, press down on the grilled cheeses with a second cast-iron skillet to mimic a panini press while they cook.
- Allow the grilled cheeses to rest for a minute or two, and then cut into square crouton-sized pieces. Reheat the soup as necessary, ladle into bowls, and garnish with a chiffonade  of basil, grilled cheese croutons, and freshly cracked black pepper.
- North American
- Serves 6