Prepare onions rings according to package instructions.
On one slice of bread, layer, fontina, onion rings, arugula and then gruyere. Top with remaining slice of bread.
Melt .5T butter in skillet over medium heat. Brush the top side of the sandwich bread with half the melted butter and then place in heated skillet. Cook 3-4 minutes, just until browned. Brush butter on the other side of sandwich and then flip over. Cook another few minutes until sandwich is golden brown and cheese is melty.
Enjoy!