Combine the mustard, brown sugar, vinegar, hot sauce, and salt in a small saucepan.
Bring to a gentle simmer over medium heat, then immediately reduce heat to low so that the sauce is barely simmering. Let it cook gently for 10 minutes.
Remove from the heat and let cool before transferring to a lidded jar or container. The sauce will thicken slightly as it cools. Store in the fridge for up to 2 weeks.
Notes
Don't let the sauce boil vigorously as it will become too thick. Adjust the heat as necessary to only maintain a very slow simmer.
This recipe is suitable for gluten-free, dairy-free, and vegan diets.