How to Make It
Preheat oven to 425°F. Bring 4 quarts water and 2 tablespoons of the salt to a boil in a large pot over high. Set 1 cup of the broccoli aside. Add remaining broccoli to pot, and cook 3 minutes. Add spinach, pushing down to submerge in water. Cook 1 minute or until broccoli is tender and spinach is wilted. Remove 1 cup cooking liquid; set aside. Drain in a colander, discarding remaining cooking liquid. Run cold water over broccoli and spinach, draining well, until cool, 2 to 3 minutes.
Process cooked, cooled broccoli and spinach, 1 cup reserved cooking liquid, stock, and parsley in a blender until smooth, 1 minute. Transfer to pot; stir in half-and-half, 2 1/2 teaspoons of the salt, and 1/2 teaspoon of the pepper.
Toss together bread, Parmesan, and 3 tablespoons of the oil. Place in 1 layer on 1 side of a greased rimmed baking sheet. Toss reserved 1 cup broccoli with remaining 1 tablespoon oil and 1/4 teaspoon each of salt and pepper. Place in a single layer on other side of baking sheet. Bake in preheated oven until broccoli is tender and bread is toasted, 7 to 9 minutes.
Gently reheat soup over medium until steaming, 4 to 5 minutes. Stir in lemon juice. Ladle soup into bowls, and top with roasted broccoli and croutons.