Preheat the broiler. Cover the potatoes with cold water in a saucepan and salt the water. Bring to a boil over medium-high heat and cook until tender, about 10 minutes. Transfer to a colander with a skimmer or slotted spoon. Add the green beans to the boiling water and cook until crisp-tender, about 3 minutes. Transfer to the colander with the potatoes, run under cold water and pat dry.
Meanwhile, mix 3 tablespoons mustard, the olive oil and celery salt in a bowl and rub over the pork. Broil on a foil-lined broiler pan until golden brown, 7 to 8 minutes; flip and broil until cooked through, 2 to 3 more minutes. Let rest 5 minutes before slicing.
Mix the mayonnaise, sour cream, the remaining 1 tablespoon mustard, the tarragon, lemon juice and zest, 1/2 teaspoon salt, and pepper to taste in a bowl. Add the potatoes, green beans and onion; season with salt and pepper, and toss. Slice the pork and serve with the potato salad.