Heat 2 tablespoons of olive oil over medium-low heat in a Dutch oven, or a heavy bottomed pan.
Add the mushrooms and ½ teaspoon of salt. Reduce the heat to low, cover the pan and allow the mushrooms to release their liquid for 5 minutes.
Uncover the pan and saute the mushrooms for a further 5 minutes or until they're fully browned. Using a slotted spoon, remove the mushrooms from the pan and transfer to bowl.
Wipe the pan out and heat 1 tablespoon of oil in the skillet over low heat. Saute the garlic, chili pepper flakes, and rosemary for 30 seconds. Take the pan off the heat and allow it to cool for 3 minutes.
Add the vinegar, maple syrup, ½ teaspoon of salt, and ¾ cup of olive oil to the pan. Stir to combine.
Return the pan to the stove and heat the mixture through over very low heat, approximately 2 minutes.
Put the mushrooms in the bottom of a 1 pint jar. Pour the liquid over the top.
Using a chopstick, stir the mixture gently, to release any air bubbles.
Put the top on the jar.