You can sit on it, or you can press it with something flat and heavy. Lan Lam includes squashing for an hour as a required step in her Simple rules for better sandwiches (Youtube), but she uses a cutting board and a pot. Pressing helps keep a big sandwich together and it also mixes the flavors a little.

Jamming it in a jar doesn’t really work for keeping it together (that burger looks like it needs to be eaten with a spoon), but it could still help disperse the flavors.