How to Make It
Make the steak In a blender, puree the gochujang with the mirin, soy sauce, sesame oil, garlic and ginger until smooth. Put the flank steak in a large resealable plastic bag and pour in the marinade. Seal the bag and refrigerate for at least 2 hours or up to 6 hours. Bring to room temperature 30 minutes before cooking.
Meanwhile, make the pasta salad In a large pot of salted boiling water, cook the fusilli until al dente. Drain and rinse under cold water until cool; drain well. In a large bowl, whisk the mayonnaise with the gochujang and lime juice until smooth. Fold in the fusilli.
Heat a large cast-iron skillet. Lightly brush the zucchini, squash and pineapple with 1 tablespoon of the oil and cook over moderate heat, turning once, until charred and tender, about 8 minutes. Transfer to a work surface and let cool. Cut the vegetables and fruit into 1/2-inch pieces and add to the pasta. Fold in the peas and scallions and season with salt. Wipe out the skillet.
Heat the skillet, then add 1 tablespoon of oil. Remove the steak from the marinade, letting the excess drip off, and cook over moderate heat, turning once, until an instant-read thermometer inserted into the thickest part registers 125° for medium-rare, 5 to 6 minutes per side.
Transfer the steak to a work surface and let rest for 5 minutes. Thinly slice and serve with the pasta salad.