Position racks in the upper and lower thirds of your oven. Line a large rimmed baking sheet with parchment paper and place it on the lower rack. Preheat the oven to 450 degrees F with the sheet pan in it.
Mix frozen Brussels sprouts with olive oil, salt, and pepper.
Once the oven is heated, remove the sheet pan from the oven and carefully pour the Brussels sprouts into the center. With a spatula, spread them out, then return the pan to the lower rack. Bake for 10-15 minutes.
Stir the Brussels sprouts, then return the pan to the lower rack (if you are using two pans, switch their positions on the upper and lower racks).
Continue baking for 10 to 15 additional minutes, until the sprouts are browned on the outside and tender on the inside. Squeeze lemon juice over the top and sprinkle with Parmesan. Enjoy!
