Melt butter in a 10-inch saute pan (preferably nonstick) over medium heat and add apples and sugar, cooking gently until caramelized and softened. Whisk together eggs, milk, flour, salt, and oil. Pour over apples and cook until edges begin to brown, about 5 minutes. Use a large plate to invert tart and slip back into the pan. Cook for about 2 more minutes and transfer onto serving plate. Garnish with powdered sugar and mint.