Whisk together the eggs and milk in a large bowl; season with salt and pepper. In a separate small bowl, mix together the feta, mascarpone and tarragon. Melt the butter in a large heavy skillet over medium heat. Add the eggs and cook until almost set, stirring occasionally. Meanwhile, heat olive oil in a separate skillet over medium. Add the garlic and arugula, and sauté until the arugula is wilted; set aside. Add the cheese mixture to the eggs and stir until the cheese is melted and the eggs are softly set. Divide the arugula among 4 plates and top with the eggs. Garnish with additional tarragon. Serves 4.