Heat olive oil and butter in a large saute pan over medium heat.

Add diced onions to the pan.

Cook onions until translucent.

Add thinly sliced potatoes to the pan.

Mix well to coat potatoes with oil and onions.

Cover the pan with a lid.

Let the onions caramelize and the potatoes soften for about 10 minutes.

Add chopped swiss chard to the pan without mixing.

Cover with the lid again.

Allow the swiss chard to steam for about 5 minutes.

Mix the contents of the pan.

Recover the pan and let it cook for another 10 minutes, stirring occasionally.

Remove the lid.

Season with minced fresh basil, salt, and pepper.

Mix well.

Cover the top with slices of swiss cheese.

Allow the cheese to melt for a minute.

Serve casserole style.