Add rice to saucepan of boiling salted water, boil uncovered for 12 minutes or until rice is tender. Drain. Combine rice, one egg, finely chopped onion and melted butter, mix well.
Press rice mixture over base and side of greased 18 x 28cm lamington tin.
Drain tuna, reserve the liquid, and press flaked tuna over rice pressing firmly.
Melt extra butter in a pan, add flour and stir until combined. Remove pan from heat, add milk and tuna liquid and stir well.
Return pan to heat and stir until sauce thickens. Add mustard, paprika, salt and pepper. Stir until combined, simmer uncovered for approx 2 minutes. Add well beaten 2nd egg, grated cheese, parsley and lemon juice. Stir until combined. Pour sauce over tuna.
Bake uncovered in moderate oven for approx 40 minutes or until firm on top.