Wrap the whole allspice in a double layer of cheesecloth. Gather the edges tightly and secure with kitchen string to create a spice sachet.
In a small bowl, stir together 3/4 cup of water, the white vinegar, and maple syrup until evenly combined. Set aside.
In a small saucepan, combine the flour, cornstarch, ground mustard, turmeric, salt, and the remaining 3 tablespoons of water. Whisk until smooth and free of lumps. Slowly pour in the vinegar-maple mixture, whisking constantly to prevent clumping.
Drop the prepared spice sachet into the saucepan. Bring the mixture to a gentle simmer over medium heat. Cook for 5 to 10 minutes, stirring frequently, until the mustard thickens to your desired consistency.
Remove and discard the spice bag. Transfer the mustard to a clean, covered container and let it cool slightly before refrigerating. Once fully chilled, the mustard is ready to use and can be stored in the fridge for up to one month.