Heat 2 tablespoons oil in a large saucepan over medium heat. Add onion and fennel and sauté 5 minutes or until slightly softened. Add garlic and mushrooms and sauté 5 minutes. Add 3 1/2 cups boiling water and stir in bouillon. Return to boil; reduce heat and simmer, uncovered, 10 minutes.
Preheat grill or broiler. Brush bread lightly on both sides with remaining oil and toast until golden, about 1 minute per side. Cut into cubes and place in a medium bowl. Add parsley and mint and toss.
Season wild mushroom soup to taste with salt and pepper. Ladle into bowls and sprinkle with croutons.