Slice the Mahi into manageable fillets so that they can be easily flipped in the pan. Pat the fish with a paper towel to remove excess moisture. Lightly dust the fish with fresh cracked black pepper and set aside. In a small sauce pan over low heat, melt the butter and combine the WannaRub. Set aside.
In a small mixing bowl whisk together the juice from the lemon, olive oil, and a pinch of salt and a pinch of pepper. Once combined toss the arugula in the mixture.
Add a thin layer of oil to the pan, about an 1/8 inch. Over medium high heat bring the pan up to about 400 degrees or just at the smoke point. Once the oil is hot add the fish and sear from 30 seconds to about 1 minute or until the fish is just cooked through. There should be a crust on the outside. Let the fish rest for a moment until any excess oil has drained. Brush the butter and Wannarub mixture on both sides of the fish. Lay the fillets over the bed of arugula and serve immediately.