In a mortar, lightly crush the garlic with a pestle. Sprinkle with 2 tsp. salt and let sit for 5 minutes. Add the oregano, cumin, and 1/2 tsp. black pepper and mash until a paste forms. Transfer to a medium bowl and add the lime juice, orange juice, and sugar; whisk until well combined.
Lay the steaks in a 9x13-inch glass or ceramic dish and pour the garlic mixture (the mojo) over them. Let the steaks marinate for no more than 10 minutes.
Meanwhile, heat the oil in a 12-inch skillet over high heat. Add the onions and peppers and cook, stirring constantly until they begin to soften, about 2 minutes. Transfer to a bowl.
Add the steaks to the pan along with 2 Tbs. of the mojo (discard the rest). Cook for 2 minutes, flip, add the onions and peppers, cover, and cook for 1 minute more. Uncover and continue cooking until the meat is just cooked through, about 2 minutes more.
Serve the steaks topped with the onions, peppers, and a drizzle of the cooked mojo.