Whisk together the fresh pineapple juice, soy sauce, cumin and cayenne. Pour over the flank steak and marinate for about 30 minutes.
Prepare your grill and cook the flank steak over low heat (flipping once) until the internal temperature of the steak reaches 155 degrees (for medium doneness). Remove the flank steak from the grill, tent with foil and allow to rest for 10 minutes before slicing to serve.
Place the whole poblano pepper on the grill and cook, flipping occasionally until the pepper's skin is blackened. Remove from grill and place the pepper in a gallon-sized Ziploc bag. Leave in the sealed bag for about 10 minutes to steam. Remove the pepper and peal the skin and remove the stem and seeds. Finely chop the grilled pepper and add to a clean bowl.
Also, grill the half of red onion and cool. Chop fine and add to the bowl with the poblano pepper.
Finely chop the fresh cilantro. Add the cilantro, lime juice, oil, honey and salt to the bowl with the grilled poblano and onion. Stir everything together to make the chimichurri sauce.
Heat a non-stick pan over high heat and add the frozen corn. Cook, stirring often, for about five minutes to char the corn.
Slice the steak and serve the tacos filled with steak, chimichurri sauce, corn and sour cream.