In a medium skillet, bring the water and olive oil to a simmer. Add the garlic in an even layer, cover and cook over moderate heat until the water has evaporated, about 5 minutes. Uncover and cook over moderate heat, undisturbed, until the garlic begins to turn golden, about 5 minutes longer. As the garlic colors, transfer it to a plate. Pour 1 tablespoon of the garlic oil into a medium bowl. Whisk in the mustard and vinegar and season with salt and pepper.