Preheat the oven to 425°F.
Preheat a 9" or 10" oven proof skillet (cast iron or stainless steel) either in the oven or on the cooktop over medium heat. (see notes)
In a medium bowl, combine eggs, milk, flour and a good pinch of salt with a fork or whisk. The consistency should be rather thin. Set aside until your skillet/oven is completely preheated.
Once ready, swirl or brush melted butter over the surface of your skillet. Carefully pour in the pancake batter and immediately place in the oven. Bake for 13 to 16 minutes until it's puffy and dark golden brown around the edges. DO NOT open the oven door while it's baking.
While the pancake is baking, fry the eggs. Add cooking fat to a nonstick skillet preheated over medium heat. Crack the eggs into a small dish one at a time before sliding them into the pan. Cook the eggs undisturbed until the whites are opaque and just cooked through. Remove the pan from the burner and leave the eggs there until assembly.
If you haven't already, prepare the cheese, ham, and radish while the eggs cook. Toss the greens with red wine vinegar and olive oil.
Very carefully remove the hot skillet from the oven and slip the pancake onto a serving dish. Immediately top with gruyere so the cheese begins to melt. Add eggs and Serrano ham. Place the mound of greens right in the center and garnish with radish. Add a pinch of salt and fresh cracked black pepper.
To serve, slice into 4 wedges so there is one whole intact egg on each wedge.