Place basmati rice (parboiled rice preferred) in a large bowl and cover with warm water. Set aside. Wash heads of garlic. Set aside.
Heat vegetable oil and margarine in a cauldron over high heat, then add mussels, stir-fry about 10 minutes. Stir in the onions; cook and stir until the onion has softened. Stir in the carrots; cook and stir until the carrots have softened. Sprinkle with pepper. Drop whole garlic heads into the mixture, stirring to evenly distribute ingredients. Reduce heat to medium. Cover and cook for 10 minutes.
Wash and drain basmati rice with hot water. Pour cleaned rice over the mussel mixture in an even layer. Slowly pour in the boiling water with salt. The rice should be covered with about 3/4 inch of water. Do not stir. Reduce heat to medium-low. Cover and cook until rice is tender, and the liquid has been absorbed, about 20 minutes.
Stir rice and mussels together, and serve with the garlic heads on top.