Combine the olive oil and balsamic in a jam jar or other container with a good-sealing lid. Add a big pinch of salt and a few grinds of black pepper. Screw on the lid and shake vigorously. Dip a piece of lettuce into the vinaigrette and taste. Adjust the salt, pepper, or the proportion of oil and vinegar to taste.
This vinaigrette will keep on the counter for several weeks (refrigerate if you added any fresh ingredients). The oil and vinegar will separate a few minutes after shaking — shake to recombine before dressing your salad.
Notes
Optional extras : spoonful of mustard, minced shallots, minced garlic, minced fresh herbs, teaspoon dried herbs, spoonful of honey or brown sugar