Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.
Mix butter, mayonnaise, garlic, oregano, salt, pepper, and sage in a bowl until well combined. Spread mixture evenly on baguette halves, then sprinkle Parmesan cheese over top.
Place baguette under the preheated broiler until lightly toasted, about 5 minutes.
