In a Dutch oven, heat olive oil and sauté the onions until soft. Break the pasta into 3 or 4 pieces and add to onions. Cook over medium heat until pasta starts to turn golden. Add the garlic, tomato paste, and chorizo and continue to cook for 3-4 minutes. Add the fennel, clam juice, and water and season with salt and pepper. Bring to a boil then cover, lower heat, and let simmer for 5 minutes.