In a large skillet, over medium heat, heat olive oil and add minced garlic. Saute garlic for 1-2 minutes, be careful not to burn the garlic or it will become bitter. If the sausage is in a casing, remove it from the casing. Crumble the sausage into the pan with the garlic. Allow it to brown and cook through. Add arugula and stir until evenly wilted, about 1-2 minutes. Transfer from a skillet to a bowl, draining away the drippings.
In a large bowl, whisk together eggs, cheese, milk, salt and pepper. Add the sausage/arugula mixture and stir to combine.
Pour into a greased baking dish. Bake at 350 degrees until eggs are set in the middle and slightly puffed, about 25-30 minutes. Let cool for 5 minutes in the pan. Carefully run a paring knife around the inside edge of the pan to release the frittata. Cut into wedges and serve warm or at room temperature.