Place beef flank on cutting board and pound to 1/8" thickness. Some cut flank but I prefer to pound it for tenderness. Set aside.
Saute onions and mushrooms (separately as the mushrooms contain more water)
Sprinkle meat with salt, add all ingredients and roll tightly tying 6-8 sections and cutting respectively. (Each round should have its own tie)
Add olive oil and butter (extra butter if you'd like it for taste) to your skillet and cook on each side for 4-6 minutes depending upon your desired tenderness and wellness.
Grass fed flank only needs to sear on each side for 2 minutes. The reason, it carries less bacteria and therefore can be eaten safely in a rare to raw state.
This also works well with a pork loin.