In a pan over medium fire, heat cooking oil.
Add the butter and let it melt completely. (We melt butter in oil to prevent it from burning)
Add garlic and sauté until fragrant.
Add onion and pepper then sauté for 1 minute.
Add the cleaned mussels. Add some salt if you want it to be tastier or it happened that you used light butter. Simmer until the shells opened.
Transfer to a serving bowl. Serve while hot and enjoy your buttered tahong!