1. Preheat the oven to 425°F. Place a 12-inch cast iron skillet in the oven while it is preheating, on the center rack.
2. Add the eggs, flour, milk, salt, pepper and dijon to a blender and blend until smooth, about 1 minute. Alternatively, you can use an immersion blender.
3. Once the oven is preheated, remove the skillet from the oven, add the butter to the hot skillet and place it back in the oven. Once butter is melted, pour the batter into the pan. Place in the oven and bake on the center rack for 18-20 minutes.
4. Remove from the oven. Sprinkle with the Havarti and prosciutto. Place the skillet back in the oven until the cheese is melted, about 5 minutes.
5. Cut the pancake into 6 wedges and serve topped with your favorite leafy greens.