Place a medium saucepan over medium heat and add the unsalted butter. Allow the butter to melt slowly, stirring occasionally.
Once the butter is melted and starts to foam, add the fresh sage leaves. Cook, stirring occasionally, until the butter turns a golden brown color and emits a nutty aroma.
Remove the saucepan from the heat. Season the sauce with salt and freshly ground black pepper to taste. Stir in lemon juice if using.
Serve the brown butter sage sauce immediately over your choice of pasta, vegetables, or proteins.