Use an outdoor grill or stovetop to heat the olive oil in the cast iron skillet until shimmering. Sauté the onions until fragrant – about 2-3 minutes, then add the artichoke hearts, garlic, tomatoes and spinach. Stir until the spinach is wilted, another 5-6 minutes.
Meanwhile, in a small bowl stir together the cream cheese with the mozzarella cheese, pepperjack cheese and half of the Parmesan cheese. Stir in the Greek yogurt then fold in the sautéed vegetables until completely combined.
Transfer the cheese mixture back to the cast iron and spread into an even layer. Arrange the sliced jalapeno around the edges of the cast iron, as desired. Sprinkle the remaining Parmesan cheese over the top of the dish.
Cover with foil. If using an outdoor grill with a lid, bake at 500 F for 10-15 minutes over a low flame. If using a standard oven, cook at 425 F for 15-20 minutes. For both methods remove the foil and continue to cook until the Parmesan is browning around the edges – another 10-15 minutes.
Let the cast iron cool for another 10 minutes prior to serving. Serve with toasted pitas or vegetables. Enjoy!