If you're still hungry after eating your in-flight meal, you can ask a flight attendant for another one for free.
Travel & Leisure notes that most airlines are happy to give you an additional meal if there's one available. Availability will, of course, vary from flight to flight, and you might not get your first pick between the chicken and the pasta, but if your stomach is still rumbling post meal there's the possibility of getting seconds.
Typically it's best to ask as soon as possible since there are likely to be limited extra meals (if any).
Flight attendants are unlikely to give you two meals at once, but will probably acknowledge your request and let you know whether there's any food left over after they finish service for the entire plane.
I actually did this last year on a 14-hour flight from San Francisco to Italy. I have a shellfish allergy which I had noted on my reservation, earning me the "food allergy" meal onboard. While everyone around me was enjoying omelets, I was given a weird breakfast "allergy meal" which I think was the meal to satisfy any allergy anyone could possibly have. It was three slices of tomato.
I had slept through the night before's dinner service and was exceptionally hungry. Flight attendants served the rest of the plane but brought me an omelet once they knew they had enough to go around.
In the end, you're still eating aeroplane food, which even when it's good aeroplane food is typically not the most delicious dining option. But it's definitely better than nothing, or shelling out cash for an overpriced can of Pringles on board.
If you know you're going to be super hungry on board, it's always good to pack some high-protein snacks to get you through. I always fly with a bag of trail mix, which has been clutch not only for an in-flight snack but also when I need a snack in my hotel at my destination.
And if you have enough time before you board, you can always grab a meal at the airport to take onboard and then ask the flight attendants to save your in-flight meal for you until you're hungry later in the flight.