Saute onion slices with a little bit of oil until golden around the edges and tender, about 5 – 7 minutes keeping an eye on the last 2 minutes so they don’t over char on the edges.
Layer both inside slices of bread with a generous amount of hummus (I used this recipe). Add a layer of sautéed onions, then sliced tomato, arugula and a sprinkle of salt & pepper. Top with a slice of bread and press down. Use the same pan, so it's preheated and lightly oiled.