Marinate flank steak for at least 6 hours up to 24 hours.

Remove the steak from the refrigerator 20 minutes prior to cooking. Prepare the guacamole while you wait to cook the meat.

Preheat the oven to broil and place the oven rack at the top of the oven. Line a baking sheet with tin foil.

Clean the Anaheim chili then remove the stem. Slice the pepper down the middle, without cutting all the way through, and remove the seeds. Flatten the chili on the baking sheet and place it in the oven to broil.

Once the skin is blistered and black, remove the chiles from the oven and place them in a bowl; cover with saran wrap and let them sit for 5-7 minutes. Carefully, peel the burned skin off of the chili. Cut the chiles into strips.

Heat a grill pan over medium-high heat and spray with cooking spray. Once the pan is really hot add the flank steak and cook for 3-4 minutes per side or until your desired degree of doneness. Place the steak onto a cutting board; let the meat sit for at least 5-7 minutes prior to slicing thinly against the grain. Slice the meat into bite-sized pieces.

While the meat rests, add the scallions to the grill pan and cook on each side for 2-3 minutes or until wilted and tender.