Preheat the oven to 400 degrees F. Line a baking sheet with foil. Spread the frozen brussels sprouts (keep them whole, no need to thaw) in a single layer on the sheet. Toss with 1 tablespoon of the olive oil and season with salt and pepper to taste. Leave room between the sprouts to allow excess moisture to steam off. Roast for 20 minutes.
Remove the sprouts from the oven and carefully transfer them to a cutting board. Slice in half lengthwise and transfer them back to the baking sheet. Toss in 1 tablespoon of olive oil and 2 tablespoon of the balsamic vinegar. Arrange the sprouts cut side down. Roast for 10 to 15 more minutes until crispy.
Before serving, toss the sprouts in the rest of the balsamic vinegar (or if you have it, balsamic reduction or glaze). Enjoy warm.
Notes
For more flavor, add some shredded parmesan cheese before serving. You can also broil the sprouts with the cheese for a few minutes to melt it.
These are best enjoyed right away, as they will lose crispiness over time.
Double the recipe if you have a 20-ounce bag of frozen brussels sprouts, but be sure to leave enough room between the sprouts on the baking sheet. Use 2 sheets if needed.