Heat vegetable oil In a small saucepan. Add garlic, onion and ginger. Saute until onion is soft.
In a small bowl, stir ketchup, miso and black pepper together until smooth. Add to saucepan. Stir in 1/4 to 1/2 cup water for desired consistency. Bring to a boil, stirring constantly. Remove from heat and serve with Tempura.