Turn on the broiler and toast the bread until golden; rub with the cut sides of the garlic, then cut into cubes and place on a foil-lined baking sheet. Drizzle with EVOO; season with sea salt, pepper and the pecorino-romano. Return the croutons to the oven and broil until the edges are crisp, 1 to 2 minutes.
In a medium soup pot, heat 3 tbsp. EVOO , three turns of the pan, over medium-high. Add the zucchini, fennel, onion and garlic. Season with salt and pepper and partially cover the pot. Cook, stirring occasionally, until vegetables soften, 10 to 12 minutes. Add the stock and a mug of water (about 1 1/2 cups). Bring to a boil, reduce to a simmer and cook until ready to serve. Stir in the herbs.
Divide the croutons among 4 bowls and top with the soup. Let stand 2 to 3 minutes, then garnish with the reserved fennel fronds before serving.