Hello there cmfulmer.
Some workout on an empty stomach without complaint. Personally, I like to have some energy to burn. It helps me feel energized so that I can give my workout the absolute most (and without my stomach growling on me

).
If time is a problem, then consider even half a meal for pre-workout. That can be in the form of a shake, a bar, or whole/natural foods. Depending on the time available between your meal and your
exercise routine, you'll want to be selective about the foods that you eat. The nutritional timing (particularly of the carbohydrate fuel) is important. Given your 15 minute timeframe, I'd recommend something fast-absorbing (like what you're currently doing).
Post workout ingestion of nutrition is most beneficial as soon as you can gulp it. A liquid shake is ideal. Typically, the faster
digesting protein, combined with a liquid (higher glycemic) carbohydrate source (, and maintaining a lower fat & fiber content) is ideal.