Preheat the broiler. Heat the olive oil in a large skillet over medium heat. Add the onion, fennel and a pinch each of salt and pepper; cook, stirring, until the vegetables are tender and just starting to brown, about 10 minutes. Add the minced garlic, oregano, olives and red pepper flakes and cook until the garlic is softened, 1 to 2 minutes. Add the vermouth and cook until almost completely evaporated, about 3 minutes. Add the tomatoes and bring to a boil. Reduce the heat to medium low and simmer until slightly thickened, about 5 minutes.
Generously season the cod with salt and pepper and nestle in the sauce. Cover and cook until the fish is mostly cooked through, 6 to 8 minutes. Turn off the heat and let the fish finish cooking in the sauce, about 3 more minutes.
Meanwhile, brush the ciabatta with olive oil and broil until toasted, 1 to 2 minutes per side. Rub with the smashed garlic.
Divide the sauce and fish among shallow bowls; drizzle with olive oil. Top with fennel fronds and serve with the ciabatta.