Add the olive oil to a medium saucepan and heat over medium heat. Add the onions and fresh thyme. Add salt and pepper and stir to combine. Stir for several minutes until onions are softened and becoming translucent.
Add in the rest of the ingredients and stir over medium/low heat until the liquid is nearly evaporated and the onions are caramelized, up to 15 minutes.
Allow jam to cool in the pan and transfer to a container. Store in fridge for up to two weeks.
For the Burgers
Form the beef into 4, 1/4 lb. patties. Dust with salt and pepper. Cook over a hot flame, flipping half way through. Once flipped, add the blue cheese to the patty and allow to cook through and the cheese to melt.
Remove the patty from the flame and allow to rest for a minute.
Slather the bottom bun with 1/2 T of mayo. Add the juicy burger and then top it with the prune onion jam. Next add the tomato and lettuce leaf and add the top bun and enjoy!