Pat the flank steak dry with a paper towel to ensure a hard sear, then season both sides with the dried oregano, half of the sea salt, and half of the black pepper.
In a small glass bowl, whisk together the finely chopped fresh parsley, minced garlic, red pepper flakes, red wine vinegar, and half of the olive oil to create the chimichurri.
Slice the zucchini into half-inch rounds and the red bell pepper into wide strips, then toss them in a bowl with the remaining olive oil and the rest of the salt and pepper.
Preheat a cast-iron grill pan over high heat until it begins to smoke slightly, then place the steak in the pan and sear for 4 to 5 minutes per side for a medium-rare finish.
Add the prepared zucchini and bell peppers to the grill pan during the last 6 minutes of cooking, turning them once until they are tender and show dark char marks.
Remove the steak from the heat and let it rest on a cutting board for at least 5 minutes to allow the juices to redistribute before slicing thinly against the grain.
Arrange the sliced steak and charred vegetables on a plate and finish by drizzling the fresh chimichurri sauce over the meat.