Place almond slivers in a dry skillet over medium heat. Heat, stirring constantly until nuts brown and become fragrant. Remove almonds from pan and transfer to a small bowl to cool.
Peel oranges and slice fruit into wedges to remove pith.
Fill a large bowl with ice water. Fill a large stock pot with water and bring to a boil. Cook green beans in boiling water until just tender (about 5-6 minutes). Use a slotted spoon to quickly remove beans from boiling water and immediately plunge them into the bowl of ice water to stop cooking. Drain ice water from green beans and set vegetables aside.
Combine oil, vinegar, mustard, salt and pepper in a bowl and whisk together until ingredients are uniformly incorporated. Toss blanched green beans and orange slices with this vinaigrette. Chill in a serving dish (covered with cling wrap if it’s going to be in the fridge for longer than a few hours) and when ready to serve, sprinkle the toasted almond slivers over the top to garnish.
Can be made up to several days ahead – wait to sprinkle top with almonds until just ready to serve.