I use cider vinegar for this. Normally, I use wine vinegar, and that certainly works, but I like this combination. I tend to look at traditional flavorways when I am improvising. Mustard was commonly used in Northern European cooking, where apples were more common than grapes, and cider vinegar more common than wine vinegar, and the flavors go well together. I'm not consistent, though - I still use olive oil!
I pour the vinegar into a measuring cup. It's convenient to mix it right there in the cup - I'm measuring as I go, and only using one container. Then I mix in 1 teaspoon of prepared mustard. Here, I used a commercial grainy Dijon - but you can use whatever kind you like. Varying the mustard itself varies the taste of the dressing - a grainy horseradish blend will be more assertive than a smooth yellow mustard. Whatever you have in the refrigerator is presumably what you like, and any will do, here.
I love that there are so many kinds of mustard. It makes it easy to give variety to simple meals. The change of condiment makes a plain cheese sandwich different. When I first lived away from New York City, local supermarkets had lots of space, and very little variety - there would be a whole aisle with only two national brands of mustard. Perfectly good themselves, mind you - but I was used to a choice! The tiny store I shopped in while growing up had about three feet of shelving for mustard - and half a dozen brands with 10-15 flavors overall. All in small jars... I understand, though, that now stores offer a greater assortment, as consumers demand more. I know that the stores I went to in New Jersey had started to carry the variety I had expected, shortly before I moved back to New York. And, if you normally only use one or two kinds, it might be fun to try something a little different.
Anyway... I mix the spoonful of mustard into the vinegar first, as it may dissolve more readily, then add oil. Pour it into the container. Again, if I'm using a container that I know holds 8 ounces, I mix slightly less than the full 2/3 of a cup in the measuring cup, and top it off in the bottle. If I'm keeping it in a jar or covered bowl, I'll mix it all in the cup.
And there you are. Again, a truly fresh dressing to put on your lovely fresh salad, made in just a few minutes.
Oh - I store this at room temperature. The oil will solidify in the refrigerator, and there is nothing in it that needs to be kept cool for short term storage. It's rather nice (in a prettier bottle than this one) on the table. That's one advantage to the emulsification caused by the mustard. It will separate over time, but you can shake a bottle, or stir a spoon in a container, and it stays together long enough to serve yourself, while a regular oil and vinegar dressing will separate instantly, and can leave you pouring oil on your salad.
Mustard Salad Dressing
1/3 c cider vinegar
1 tsp. prepared mustard
2/3 cup olive oil
Mix all ingredients, in order.