Heat oil in a large non-stick frying pan over medium heat. Add onion, fennel and garlic. Cook, stirring occasionally, until vegetables are soft, 5 minutes. Add tomatoes with their juice, the fish stock, fennel seeds and orange zest and juice. Season with salt and pepper. Bring to the boil, stirring. Reduce heat to low and partially cover pan. Simmer 12 minutes.
Meanwhile, cook rice according to package directions; add crushed saffron to the boiling water.
Bring tomato sauce back to the boil. Place prawns on top of sauce. Cover pan tightly; cook over low heat until prawns are done, 3 to 4 minutes.
Divide rice among serving bowls. Top with prawns and tomato sauce. Sprinkle with basil leaves, if desired.