Melt the butter in the microwave and let it cool to room temperature. Remove the skin from the mentaiko and discard it. Grate the garlic.
In a bowl, combine the butter, mentaiko, garlic, mayonnaise, and grated Parmesan cheese, and mix well.
Cut the baguette in half horizontally and spread the mentaiko paste over the cut sides. Toast in a toaster oven for 5–6 minutes until lightly browned. Sprinkle with nori seaweed and serve immediately.