Simmer green beans in salted water until barely tender and bright green, 2 to 3 minutes. Remove with a slotted spoon or wire-mesh skimmer, and transfer to a bowl of ice water. Drain, and pat dry. Add potatoes to the same pot of salted water, and simmer until tender, about 12 minutes. Drain potatoes, and halve them. Whisk together olive oil, lemon juice, mustard, and thyme in a large bowl. Add potatoes, beans, and red onion. Gently toss. Season with salt.