Line a rimmed baking sheet with foil and then rub with a bit of avocado oil to prevent sticking. Lay the steak on the prepared sheet rub with the gochujang, soy, and sweetener. Broil on high for 4-5 minutes per side. Remove from oven and allow the steaks to rest for 10 minutes before slicing against the grain.
While the steak cooks, prepare the zucchini. Heat the oil in a large nonstick pan over high heat. Add the zucchini, cut side down. Cook, stirring occasionally until lightly charred, about 5-8 minutes. Add the garlic and cook until fragrant, about 1 minute more. Add the soy and sweetener and cook until no liquid remains, about 1 minute more. Off heat, drizzle with sesame oil.
Serve the zucchini with the steak and sprinkle both with green onions and chili flakes, if desired.