Set up your Big Green Egg for indirect cooking. Preheat the dome temperature to 350°F.
Place a 12-inch cast iron skillet on the cooking grate while the Egg preheats.
Add the olive oil to the hot skillet.
Add the ground beef to the skillet. Break the beef apart with a spatula as it cooks.
Cook until the beef is fully browned and cooked through. Remove the cooked beef to a clean bowl and set aside.
Add the diced onion and minced garlic to the skillet. Cook for 2 to 3 minutes until the onions soften.
Add the fajita-style seasoning and both cans of Rotel. Stir well to combine. Let the mixture cook for another minute.
Add the cubed Velveeta a few pieces at a time. Stir as the cheese melts into the tomato mixture. Continue adding Velveeta until fully melted and smooth.
Add the shredded Monterey Jack cheese one small handful at a time. Stir between each addition so the cheese melts evenly.
Once the queso is smooth and creamy, fold the cooked beef back into the skillet. Let everything warm through for another 1 to 2 minutes.
Remove the skillet from the Big Green Egg. Garnish with fresh chopped cilantro.