The secret to this soup is having lots of different vegetables diced small. They can, of course, vary according to the season.
Pour oil into a large pot with onions, celery, and fennel. Sauté for 12 minutes on medium-low heat.
Add bay leaf, Herbes de Provence, pepper, and tomatoes and cook for five minutes more.
Meanwhile, chop vegetables into ½-inch dice. Add water and prepared vegetables to the pot. Increase heat to maximum and bring to a boil. Reduce heat and simmer for 20 minutes.
Remove from heat and let rest 5-10 minutes. Incorporate pesto. Taste, then add salt and pepper. Garnish with fennel leaves and serve immediately.