Combine mustard seeds, water, and lemon juice in a glass or ceramic bowl. Soak overnight.
Place in a blender your mustard mixture, sea salt, turmeric, and maple syrup (optional) and blend until desired consistency is reached. I like my mustard quite corse, so I don't blend much. The longer you blend, the smoother it gets.
Store in an air-tight jar in a fridge for several weeks.
Notes
Soaking time is not included in overall cooking time. (12 hours)