Green Beans with Balsamic glazed Onions

In a large sauce pan of boiling water, blanch the fresh onions for 1 minute. Drain and refresh under cold running water, drain. Using a small sharp knife, trim the root ends and slip off the skins. Preheat the oven to 400 degrees. In a small nonreactive saucepan, combine 1/4 cup of the balsamic vinegar, the butter, vegetable oil, thyme, 1 teaspoon of the pepper and 1/2 teaspoon of the salt and stir over moderately low heat until the butter is melted. In a medium bowl, toss the fresh or frozen onions with the vinegar mixture to coat. Spread the onions in a single layer on a baking sheet and roast for 35 to 40 minutes, stirring often, until evenly browned. Meanwhile, in a large pot of boiling salted water, blanch the greens beans until just tender, about 4 minutes. Drain and refresh under cold running water, drain and set aside. In a large bowl combine the olive oil, mustard and the remaining 1/4 cup balsamic vinegar and 1/2 teaspoon each of salt and pepper. Add the green beans and the roasted onions to the dressing in the bowl and toss well. Transfer the vegetables to a large casserole. Preheat the oven to 350 degrees. Cover and bake the green beans and onions for about 20 minutes, or until heated through.