Whisk water, mustard powder, salt, turmeric, garlic, and paprika in a nonreactive saucepan until smooth
Cook over medium-low heat, stirring often, until thickened into a paste (30–45 minutes)
Keep the kitchen ventilated while cooking due to strong mustard fumes
Whisk in vinegar and continue cooking until slightly thinner than store-bought mustard
Remove from heat and let cool for 7–15 minutes
Transfer to an airtight container and refrigerate up to 3 months
Allow the mustard to mellow in flavor over several days before using