In a large saucepan, add onions, garlic, jalapeño, cumin, tomatillo, sugar, salt, pepper and cilantro. Bring to a simmer and cook until tomatillos are soft, about 10 minutes. Using an immersion blender, puree until smooth. Turn heat to medium and whisk in cornmeal. When sauce has thickened, turn heat to low, add beef and stir. Stir in corn and set aside.
For the arepas: In a large bowl, whisk cornmeal, cheese, flour, salt, pepper and green onions. Add the warm milk and butter. Stir until a thick batter forms. Let rest about 10 to 15 minutes to form into a soft dough.
Form into 2-inch balls. Slightly wet your hand, shaking off any excess water. Flatten ball to 1/2 inch by gently tossing it back and forth from one hand to another. In a cast-iron pan, heat 2 tablespoons of oil. Pan fry the discs until golden brown, about 1 to 2 minutes on each side.
Place an arepa on a plate and top with shredded beef mixture, cojita cheese and cilantro.