Rinse rice with cold water; drain; set aside. Bring the chicken stock to the boil in a saucepan over medium-high heat, reduce the heat to low, cover and keep warm. Work the garlic, parsley, curry powder, saffron threads, salt, black pepper and 125ml of the hot chicken stock together with a mortar and pestle until a smooth liquid forms; set the seasoning liquid aside.
Heat the oil in a paella pan over medium-high heat; cook and stir the onion until lightly browned. Stir in the chicken, prawns, lobster and clams; cook and stir until the chicken is no longer pink in the centre, about 10 minutes. Pour in the seasoning liquid.
Stir in the rice, pour in the hot chicken stock and simmer until the rice is nearly tender, about 15 minutes. Add mushrooms, peas and mussels; stir two times and simmer for 10 minutes. Remove from heat; cover and let stand until rice is soft and flaky, about 7 minutes.