Ideally, prepare it the day before or several hours in advance so it cools completely. You can make it a day or two in advance and keep it in the fridge in an airtight container.
With a sharp knife, dice 1.1 pounds roast beef or other stewing meat into small cubes, roughly the size of chickpeas.
In a large skillet or pot, heat 3 tablespoons neutral oil over medium heat. Add 2 cups finely chopped onion, ½ cup finely chopped red bell pepper, ¼ cup finely chopped green onions and 1 large garlic clove, minced. Cook slowly until the onion is translucent and lightly golden (about 10-15 minutes). This is key for flavor.
Increase the heat and add the diced beef, overlapping as little as possible. Don’t touch or move until it’s seared, then stir to brown it on all sides. Don't overcook it at this stage; it should be pink inside, so it doesn't dry out when baked.
Add 1 teaspoon sweet paprika, ½ teaspoon crushed red pepper flakes, and ¼ teaspoon ground cumin, 1 teaspoon salt and ⅛ teaspoon freshly ground black pepper. Mix well and cook for a couple more minutes to let the flavors meld.
Taste and adjust seasoning if necessary. If the filling looks too dry, you can add 1 tablespoon beef broth or water. It's not always needed.
Remove the skillet from the heat, stir in ⅓ cup chopped green olives and let it cool completely. Transfer the filling to a wide, shallow dish so it cools faster. Cover with plastic wrap, pressing it directly onto the surface of the filling to prevent a skin from forming. It is crucial that the filling is completely cold before assembling the empanadas so the dough doesn't get soggy and is easier to work with. Ideally, let it chill in the refrigerator for at least 4 hours, or even better, overnight.