Thirty minutes is too short for a large meal. If you ate a proper meal earlier, you may need nothing else. If you are hungry or have not eaten for several hours, a small amount of carbohydrate can make the start of training more comfortable.
The short answer
With only 30 minutes available, a small familiar low-fat snack with easily digested carbohydrate is usually better than a full meal.
Practical options
A banana, a few dates with water, toast with honey, fruit puree, or a small sports drink can work. Keep the amount small and never test a new product before an important session.
Is protein required?
If daily protein and surrounding meals are adequate, protein in this exact 30-minute window is not mandatory. A little yogurt or milk may fit if tolerated.
When to skip food
If eating close causes nausea, reflux, or heaviness and an earlier meal was adequate, do not force it. Diabetes or glucose-lowering medication requires personalized advice.
Practical checklist
Do not test unfamiliar food on an important training or competition day.
Adjust amount and timing to gut tolerance and session goals.
The quality of the whole daily diet matters more than one meal.
References
Nutrition and Athletic Performance — Academy of Nutrition and Dietetics, Dietitians of Canada and ACSM — https://pubmed.ncbi.nlm.nih.gov/26920240/
ISSN Position Stand: Nutrient Timing — https://pubmed.ncbi.nlm.nih.gov/28919842/