Slice the onion into 1/8 inch rings. In a pot over medium heat add the butter and olive oil. Add the onions and cook until the onions start to soften, about 5 minutes (do not cook the onions at too high heat or they will burn). Add the porter and cook until the beer has evaporated and the onions are a dark golden color, about 15 minutes.
In a small food processor add the goat cheese, cornstarch and IPA, blend until smooth.
In a cast iron skillet melt the butter, add the potatoes, sprinkle with salt and pepper, cook until the potatoes have browned.
Preheat oven to 350.
Roll the pastry crust out to a 10 inch circle, transfer to a baking sheet that has been covered with a Silpat or parchment paper.
Spread the beer goat cheese evenly across the tart, avoiding the outer 1 inch edge.
Top the cheese with caramelized onions then with the potatoes.
Fold the outer edge up over the filling of the tart.
Bake at 350 until the crust has turned golden brown, about 20 minutes.
Top with arugula before serving.