Preheat the oven to 400F.

Bring a saucepan of water to a boil. Meanwhile, cut off the root end of the onions. When the water comes to a boil, drop the onions in and let it boil for 1-2 minutes, depending on the size. It shouldn't be soft but cooked just enough where the skin falls off. Drain the onions and let it cool. Once it's cool enough to handle, peel the skins off the onion.

Heat an ovenproof pan over high heat with butter and oil. Once hot, add the onions and thyme in one layer. Place the pan into the oven and cook for 30-40 minutes. Flip the onions even 10 minutes to cook both sides. The onions should be golden brown and tender. Season with salt and pepper.

In the same pan the onions were cooked in, add the butter and minced garlic. Cook over medium heat just until the garlic is fragrant. Add the flour and stir to combine. Stream in the wine and stock and bring to a simmer. Once it thickens, whisk in the mustard and season with salt and pepper.

Serve the onions with the gravy immediately.