Place potatoes in a medium pot and cover with cold water. Bring to a boil then reduce heat to a simmer, about 10 minutes or until tender. Drain then place in a medium-size mixing bowl.
While the potatoes are cooking, add oil to a large skillet over medium-high heat and add onions and garlic. Season with salt and pepper.
Cook until the onions are tender, about 5 minutes.
Remove pan from the heat and add mustard, vinegars, and olive oil, whisking vigorously to incorporate the dressing.
Pour the dressing over the potatoes and add the spinach, tossing to coat the potatoes and lightly wilt the spinach.