Add the green beans to a pot of boiling salted water. Cook until just tender, about 5 minutes. Drain.
Season the pork with 1/4 tsp. salt and pepper. In a large skillet heat the oil over medium-high heat. Add the pork and cook about 10 minutes or until pork is just pink in the center (145°F), turning once. Remove pork.
In a small bowl, whisk together the cider, mustard, and cornstarch. Add to the pan. Cook, stirring to release browned bits, until mixture is bubbling and thickened. Cook 1 minute more. Spoon the sauce over the pork and serve with the potatoes and beans.
Peel potatoes; cut into 1-in. chunks. Boil in salted water until tender, about 10 minutes. Drain; mash. Transfer 2 cups to an airtight container. Reserve remaining 3 cups to use in Weekend Prepped Mashed Potatoes with Butter and Milk.
Stir 1/2 cup milk and 2 Tbs. unsalted butter into potatoes. Season with 1/2 tsp. salt. Transfer to an airtight container. Refrigerate.