Set the oven to 200 degrees Celsius (390 F).
Cook the rice according to instructions, set aside and let cool to room temperature. This dish is perfect to use up your left over rice, either straight from the fridge or if it is frozen leave it on the counter for a few hours to come to room temperature.
Open the cans of tuna and discard the sunflower oil, put the tuna in a bowl and use a fork to break down in to smaller pieces.
Shred the cheese.
Open and drain the can of corn.
Prepare the white sauce
Melt butter in a saucepan, add the flour and let cook together while stirring.
Peel and mince the garlic and add into the saucepan with the milk.
Add the Dijon mustard and whisk everything together.
Bring everything to boil to let the sauce thicken and then let simmer for 10 minutes, while stirring frequently to avoid burning at the bottom.
If the sauce feels too thick, whisk in more milk. It should be the consistency of a loose gravy.
Set aside and let cool a bit.
Take out a 21cm x 21cm (8 x 8 inch) oven safe glass ware.
Start layer the tuna bake with a thin layer of sauce on the bottom, to avoid the rest of the ingredients to stick to the bottom of the glass ware.
Add rice in a thin layer and half of the tuna.
Spread out half of the corn.
Cover everything with half of the white sauce.
For the second round of layers, start with tuna, then add corn, rice and lastly the last of the sauce.
Spread the shredded cheese on top of everything.
Bake in the middle of the oven for 20 minutes at 200 degrees Celsius (390F).
If the tuna bake browns too early, cover it with aluminum foil for the last 5 minutes.
You want your tuna bake to be piping hot all the way through, with a gooey, melted cheese cover.