In a small bowl, whisk together the olive oil, lime juice, cumin, garlic powder, sea salt, and black pepper to create a marinade.
Place the flank steak in a shallow dish, coat thoroughly with the marinade, and let it sit for at least 15 minutes to absorb the flavors.
Heat a cast-iron skillet over medium-high heat and sear the steak for 4-5 minutes per side until a deep brown crust forms.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain into bite-sized strips.
Briefly toast the corn tortillas in the same skillet until they are pliable and slightly charred.
Fill each tortilla with the sliced steak, then top with diced white onion, fresh cilantro, and a sprinkle of crumbly queso fresco.
For the beverage, squeeze the juice of one lemon into a glass, stir in the monk fruit sweetener and water, and serve over ice.