This makes enough to dress a small salad, and takes a minute at most. The mustard is present as an emulsifier, as well as for flavour. You can omit it, but then the dressing will separate quickly (which doesn't really matter at all).
Put a small splash of vinegar in a jar with a screwtop lid. An old mustard jar is about the right size; I use one of those miniature jam-jars you get with airline meals. Add about twice as much olive oil as you have of vinegar. Add the tip of a teaspoon of mustard, and some salt and pepper. Screw the lid on (or cover the mouth of the jar with your hand), and shake vigourously until the dressing is emulsified. Check the flavour; if it's too tart, then add more oil and shake again.
There are all kinds of changes you can ring on this. You can substitute lemon juice for vinegar; you can add a drip of honey; or you can add a dollop of sour cream and some dill, to get a creamy dressing that works well with a cucumber salad (if you're using sour cream you don't need mustard).