In a mixing bowl, combine soy sauce, mirin, sake (or white wine), brown sugar, honey, minced garlic, grated ginger, sesame oil, rice vinegar, tomato paste, minced green onion, mustard powder, and black pepper.

If you choose to use miso paste, dissolve it in a small amount of warm water and add it to the mixture. This adds a layer of umami and depth to the sauce.

Whisk all the ingredients together until the sugar is fully dissolved, and the sauce achieves a smooth consistency. Ensure that the tomato paste is well incorporated.

Taste the sauce and adjust the flavors according to your preference. You can add more honey for sweetness, soy sauce for saltiness, or a splash of sake for depth.

If desired, set aside a portion of the sauce for marinating meats or vegetables. Marinate for at least 30 minutes for enhanced flavor.

Brush the sauce onto grilled meats during the last few minutes of cooking, or use it as a glaze. It can also be used for baking or pan-frying.

For a thicker consistency, transfer a portion of the sauce to a small saucepan. Simmer over low heat until it reaches the desired thickness. Stir occasionally to prevent burning.

Allow the sauce to cool before serving. This helps it thicken slightly and intensifies the flavors.

Store any unused sauce in an airtight container in the refrigerator. It can be stored for a few weeks, and the flavors may mature over time.

Serve the Japanese Fusion BBQ Sauce with grilled meats, vegetables, or as a dipping sauce for a unique fusion twist.