Prepare the dashi stock by dissolving 1 teaspoon of instant dashi powder in 1 cup of hot water, or use freshly made dashi if available.In a small saucepan, combine the dashi stock, soy sauce, mirin, and sugar. Stir until the sugar is fully dissolved.Place the saucepan over medium heat and bring the mixture to a gentle simmer. Do not allow it to boil.Taste the sauce and adjust sweetness or saltiness if needed.Remove from heat and serve warm or at room temperature.Garnish with grated daikon radish and sliced green onions if desired. Serve alongside crispy tempura.