Preheat oven to 400ºF.
Place the pork tenderloins onto a baking sheet and coat them with 2 tablespoons of EVOO, grill seasoning and grainy mustard, rubbing to evenly coat. Transfer to the oven and roast until cooked to your desired doneness, about 25-30 minutes or until a thermometer inserted into the pork reaches 145ºF. Let the meat rest about 5 minutes once it comes out of the oven.
While the pork is roasting, place the potatoes into a medium-size saucepan and cover them with cold water. Add some salt and place over medium-high heat to bring to a bubble. Reduce the heat to medium and simmer until tender, about 15 minutes. Drain the cooked potatoes and return them to the pot they were cooked in. Add the butter and milk and mash the potatoes.