Preheat the oven to 425˚F. Line a baking sheet with parchment paper or foil.
Add the butter to a small skillet and melt over medium-low heat. Add the garlic and lemon juice and reduce the temperature to low. Continue cooking until the mixture thickens slightly and is fragrant. Remove the butter from the heat.
In a large bowl, add the brussels sprouts and pour butter over them. Toss the sprouts to evenly coat them in the butter. Arrange the sprouts in an even layer on the prepared baking sheet. Sprinkle with salt and pepper.
Roast the sprouts for 18 minutes, flipping them halfway through. Allow to cool for 3-5 minutes before serving.
Top with chopped walnuts for extra crunch, if desired.