The Double Cross
Preheat your oven to 300 degrees.
After removing the stems, rub olive oil onto the top of the mushrooms and season with sea salt. Place the mushrooms gill side up on top of a wire rack (I used a cookie cooling rack) which is itself on top of a cooking sheet that has been covered in foil (I did this simply to make cleanup easier). Drizzle the mushrooms with a little more olive oil and season with another sprinkle of sea salt.
Put them in the oven and set your timer to 15 minutes. As they cook, the mushrooms with exude moisture (mushrooms are mostly water by weight), so you want to flip them every 15 minutes for an hour or more. The wire rack will allow air to flow around the mushrooms which will leave the caps firm and dry rather than slimy.
When the mushrooms are done set on a paper towel to further dry them out.
Note: You can also use wire rack (with your oven set to 200 degrees) to keep your bacon hot while preparing other ingredients.
In a large frying pan, cook 4 slices of bacon over medium heat. Cook the bacon until crispy. Remove the bacon to a paper towel to dry, but leave the bacon grease in the pan.
Reduce the temperature to medium-low and add your onions to the pan. Allow them to completely caramelize so that they are completely soft and golden brown. This may take ~20 minutes, but you can simply turn the heat to low and cover to keep them warm while you are working on other ingredients.
To make the mashed avocado, simply slice an avocado in half by running a sharp knife through the center. Twist the two halves to separate. Using the tines of a fork, mash the meat of the avocado while still in it’s skin. If you are going to leave the avocado out for awhile, drizzle with a little lime juice to prevent browning.
Note: There is sufficient fat in the beef so no additional greasing of the pan is required.
Heat a skillet to medium heat (I used iron, but any oven safe cookware will do) and turn on your oven’s broiler.
Season the beef patties on both sides with sea salt and black pepper and add to the skillet. Place under the broiler for 3-5 minutes before flipping. Cook for an additional 3-5 minutes and then remove to a paper towel to drain slightly before assembling the burger.
On one of the portobello mushroom caps stack your lettuce, tomato, bacon and onions. On the other half, stack the beef patties and mashed avocado. With Houdini like skills, marry the two halves together. Ta-da! Prepare to be Double Crossed!