Easy one from my mom… Trim any visible fat off the flank steak and put it in a dish or freezer bag. Mix up half and half lemon juice or vinegar and olive oil (maybe 2 Tbs each, you don’t need a lot). Add a minced garlic clove, 1/4 cup fresh or 1/8 cup dried parsley, salt and pepper. Marinate the steak in this for a few hours. Bbq about 4-6 minutes per side. Let rest a few minutes. Slice thinly against the grain.