Peel the carrots and cut them crosswise into ¼ in (5 mm) slices. Remove damaged outer layers from the leek, slice as for the carrots, and rinse.
Warm a little olive oil in a Dutch oven over medium heat. Add the carrots and leek, season with salt, and stir to coat. Stir in the oregano and let cook for 5 minutes.
Add the chicken pieces and onion, stir, and cook for 5–7 minutes, until the chicken is lightly browned. Add the garlic, water, and crumbled bouillon cubes, then bring to a simmer. Simmer gently over low heat for 20–25 minutes.
Remove from the heat and stir in the red and white haricot beans, followed by the scallions and spinach. Season to taste and set aside.
To prepare the croutons, preheat the oven to 430°F (220°C/Gas Mark 7). Cut the bread into large cubes. Place them in a bowl, drizzle with olive oil, and toss to coat. Sprinkle with fleur de sel, pepper, your choice of herbs and/or spices, and stir in the Parmesan. Spread over a baking sheet and bake for 10–12 minutes, stirring halfway so they colour on all sides, until crisp and golden brown.
To serve, reheat the soup, ladle generous servings into soup plates, and add 1–2 teaspoons pesto to each serving. Scatter over the croutons, grate some Parmesan over the top, and serve.