Heat 1 tbsp butter over medium heat until fully melted

Add in onions, stirring every few minutes until they become deep golden brown, around 30-35 min

Add red wine to deglaze the pan, scraping all the bits off the bottom.

Season with salt and pepper and let cool slightly

Spread mayo on one side of each piece of bread

Put onions and gruyere on the unmayoed side, and top with another piece of bread

Melt remain butter in a skillet. Put your sandwich in when the pan is very hot

Flip and brown the other side