Melt the butter in a large frying pan over medium heat. Add the onions and stir to coat in the hot butter. Allow them to cook in the pan until very lightly golden and they start smelling a bit sweet, about 30 minutes, stirring every 5 minutes. Remove them from the pan and set them aside.

Add the olive oil to the pan and then add the frozen Brussels sprouts, taking care not to add the spice or bacon packets. Cover the pan and allow them to cook for 10 minutes.

Remove the cover and raise the heat to medium high. Add the sherry vinegar, brown sugar, sea salt, onion powder, and black pepper and stir to coat the Brussels sprouts in the mixture. Allow them to cook until they start turning a slightly olive shade of green, about 15 minutes, stirring every 5 minutes. Add the onions and the bacon bits to the pan and continue cooking until the onions are a deep rich gold and the Brussels sprouts are a deep olive green, about 10 minutes. Remove from heat, toss with the fresh thyme, and serve.