In medium saucepan, combine onions, 4 tablespoons vinegar, butter, vegetable oil, thyme, pepper and 1/2 teaspoon salt.
Heat over low heat until butter is melted; stirring to coat onions. Place mixture on a baking sheet and roast in a 400 degree F oven for 35-40 minutes, stirring occasionally until onions are browned nicely; remove from oven and set aside.
Blanch green beans in large saucepan of boiling water just until tender, about 5 minutes. Drain and rinse with cold water; set aside. In small bowl, whisk together olive oil, mustard, sugar, the remaining 4 tablespoons of the vinegar, and the remaining 1/2 teaspoon of salt.
In a large bowl, toss the dressing together with the onions and the green beans. Place the mixture in a large casserole dish and cover.* Bake for 20 minutes in a 350 degree F oven.
*Recipe can be made ahead at this point and refrigerated for a day. Bring to room temperature before cooking.