Preheat the oven to 200C/400F and leave mushroom caps inside for 30 minutes.
While mushrooms are in the oven, heat the skillet and cook the simple burger from the ground beef.
Mince the onion and gently sear it right after.
Now use a grater to get the tomato meat, without taking the skin off. Make a "X" cut and grate it - you will see all the insides going through the grater, while the skin will stay in your hands. You can also use hot water instead.
Chop parsley and mix it with onions in the tomato puree. Add ketchup, salt & pepper.
Slice the pickle. Assemble the burger, putting everything inside. Enjoy!