Heat a large pan over medium high heat and add oil and onion. Cook until the onions are soft and translucent, about 5-10 minutes. Add the garlic and cook until fragrant, about 1 minute more. Add the curry paste and cook until fragrant and darker in color, another 1 minute. Add the coconut milk and bring to a boil. Simmer for 10-20 minutes, or until reduced by half and thickened slightly. Stir in fish sauce, stevia, salt, lime juice, and zest. Adjust seasonings to taste.

While the sauce reduces, cook the steak. 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 and rub with coconut aminos. 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.