¾ cup olive oil
3TB lemon juice or ¼ cup red wine vinegar
salt and freshly-ground black pepper to taste.
a pinch of shallots, minced

Make the recipe twice and store both separately in the refrigerator.
Use one up and as soon as you need to open the second, make another recipe.
The opening of the second container is your fail-safe way of never running out.
Use it for salads but also over broiled or roasted items.