Heat the oil in a 5-quart saucepan over medium heat. Add the onion, fennel, carrot, and bell pepper and cook, stirring occasionally, until the vegetables are tender but not browned, about 5 minutes. Add the tomato paste and minced garlic and cook, stirring constantly, until the garlic is fragrant, about 45 seconds. Add the wine, bring to a simmer, and cook until the liquid has reduced by half, about 2 minutes. Add 3-1/2 cups of water, the chickpeas, thyme, pimentón, saffron, and bay leaf. Bring to a simmer, cover, and cook until the vegetables are tender and the stew has thickened slightly, about 25 minutes. Season to taste with salt and pepper.
Position a rack 6 inches from the broiler and heat the broiler on high. Put the bread slices on a rimmed baking sheet and brush both sides with oil. Broil, flipping once, until both sides are golden-brown, about 4 minutes total. Remove from the oven and rub each slice with the whole clove of garlic.
Gently stir the halibut and mussels into the stew, cover, and simmer until all the mussels have opened and the fish is cooked through, 4 to 8 minutes. Discard any mussels that do not open. Ladle into wide, shallow bowls and serve with the garlic toasts.