Place diced potatoes in a medium saucepan. Cove with cold water. Bring to a boil and cook until tender when pierced with a fork. Drain immediately.
Meanwhile, heat 2 tablespoons olive oil in a large skillet over medium-low heat. Add sliced onions and brown sugar, stir occasionally, until the onions caramelize and turn a deep golden brown, bout 10-12 minutes.
In a small skillet over medium heat, add remaining 2 tablespoons olive oil. Add sliced sage leaves and saute until lightly crisp. Remove from heat.
Using a hand-held potato masher or an electric mixer, mash potatoes to desired consistency. Add the caramelized onions and the crisp sage leaves in olive oil to the potatoes. Add coarse sea salt and freshly ground black pepper. Stir until well blended. Drizzle with extra virgin olive oil and sprinkle with a little sea salt before serving.