This is how those plastic squeeze bottles of mustard work out for us. The first 98% of the mustard squirts out perfectly. With the last little bit, we slap the bottom of the upside down bottle while simultaneously squeezing. The result is a spray of mustard all over the plate as well as the sandwich, hot dog or burger. But I can't just throw the bottle out while containing a small amount of product still. So that almost empty bottle goes to the back of the fridge, and we get a new bottle out to use.
To salvage that last 1% or 2% of mustard, I pour in about 1 tablespoon of vinegar or lemon juice, put the lid on and shake it vigorously. I then take the lid back off and turn it upside down over a small dish or tiny canning jar.
Why vinegar or lemon juice and not water? The primary flavor I taste in mustard is vinegar. (Vinegar is even the first ingredient on the label.) If I added water, I would be diluting that tangy taste and acidity. I use this salvaged mustard in homemade salad dressings, sandwich spreads, and marinades. So the vinegar or lemon juice is actually a plus in flavor and ability to break down meat fibers if used as a marinade.
Prepared mustard doesn't contain added emulsifiers, hence the need to shake the bottle before each use. Otherwise, the solids in mustard settle and leave a thin liquid at the top. In draining the salvaged mustard, this is much more obvious. The solids or thicker part sits in a mound surrounded by mustard-colored vinegar.
When I make a salad dressing or marinade with this, I try to pour off the liquid to use as the vinegar part of a mixture. I then use the thicker part in a mustard-mayo sandwich spread blend.
After draining the bottle for about an hour, I have about 2 tablespoons of useable product. I figure I've gotten near all of it out, and at that point I rinse the bottle with water to clean and recycle. And now, I've gotten rid of one more space-hogging item from our refrigerator.