Heat olive oil and butter in a medium saucepan over medium heat. Add the onions and sugar, and cook until the onions are golden brown and caramelized. Add the garlic and cook for 2 minutes.
While the onions are cooking, place the potatoes in a medium saucepan filled with cold salted water. Bring to a boil and cook until the potatoes are just cooked through. Drain the potatoes and set aside.
Add the clam broth to the onions and bring to a simmer. Add the oysters and the potatoes and simmer just until the oysters have cooked through, about 2 to 3 minutes. Whisk in the creme fraiche and season with salt and pepper. Remove from the heat and stir in the tarragon. Serve with Winter Green Salad and Green Apple Vinaigrette.
Winter Green Salad and Green Apple Vinaigrette:
Combine greens, celery, apples, and cheese in a large bowl. Whisk together both vinegars and the shallot. Slowly whisk in the oil, and season with salt and pepper. Toss the salad ingredients with some of the vinaigrette.