Combine the brown rice and water in a small saucepan. Bring to a boil, reduce heat to low and cook until all the water is absorbed, about 25 minutes.
Melt the butter with the olive oil in a frypan over medium heat; cook the garlic in the butter and oil until fragrant, 1 to 2 minutes. Pour in the wine and lemon juice; reduce heat to medium-low and simmer.
Stir in the prawns and cook until they turn pink, stirring regularly, 5 to 7 minutes. Sprinkle the parsley over and cook another 2 minutes. Add the cornflour to the liquid and stir until it thickens, about 1 minute more. Serve hot over the brown rice.