In a medium bowl, combine the tahini paste and white miso. Using a whisk or a fork, mix them until well combined. You should have a thick yet smooth base. Incorporate the maple syrup and lemon juice, soy sauce and chilli powder to the mixture. Begin adding the water, one tablespoon at a time, whisking constantly until you reach your desired consistency. You can adjust the amount of water based on how thick or thin you want your sauce to be.
Adjust the seasoning if necessary. You might want to add a bit more lemon juice for acidity or a touch more maple syrup for sweetness, depending on your taste preference.