Preheat the oven to 425 degrees F. Put your cast iron in the oven to get piping hot.
To a blender, add the 2 tablespoons melted butter, along with the flour, milk, eggs, salt, vanilla and sugar. Place the top on the blender and blend on high until completely smooth, about 30 seconds. Let come to room temperature.
Place grated parmesan cheese and herbs de Provence in a bowl and mix.
Remove the skillet from the oven. add the 4 tbsp of butter and melt. Working quickly, add the batter to the skillet, then evenly top with the cheese and herbs. Transfer the skillet to the oven immediately. Cook until puffed and golden brown, 20 to 25 minutes. (Do not open the door during the first 15 minutes, preferably not at all, or the pancake could deflate.) Tip, where you put the cheese will not puff up, so be careful how much you put.
Meanwhile, add the olive oil to a nonstick skillet and heat over medium-high heat. Add the onions and sauté until caramelized add your garlic and sauté for 5 minutes, then add balsamic vinegar and cook down.
Remove the Dutch baby from the oven and top with the onion mixture top with goat cheese, pop back into the oven for for a few minutes. Cut into wedges top and serve immediately.