If the baguette is longer than your baking sheet, cut it in half crosswise first. Then slice it lengthwise down the middle so you have two long halves with the cut side facing up.
Mix the 4 tablespoons butter, 2-3 garlic cloves, handful of parsley, and pinch of salt together in a small bowl until combined.
Spread the garlic butter generously and evenly across the cut sides of the baguette, all the way to the edges.
Place cut-side up on a baking sheet. Broil or bake at 400°F for 5-8 minutes, until golden and crisp at the edges.
If using grated parmesan, sprinkle it on top for the last 2 minutes of baking until melted.