Dice mushrooms into small pieces.  Spray a skillet with cooking spray; heat skillet over medium heat and add mushrooms.  Sauté for 2-3 minutes, or less, to allow mushrooms to soften up a bit.  Remove from heat and allow to cool completely.  (An essential step because you do not want to mix a heated food item with raw meat.  Read more here). 

While the mushrooms cool, heat the grill you will be using to cook your burgers.  This can be as simple as using a griddle pan on the stove or an outdoor grill over hot coals.  

Once mushrooms are cool, toss into a bowl with the ground beef, season with 1/4tsp salt and 1/4tsp pepper and shape into hamburger patties.  Transfer to your desired cooking location and cook until browned on both sides, no longer pink inside.  Transfer to a clean platter and cover if not ready to serve yet.  

To prepare salad:

Dice pepper, cube feta and watermelon.  Coarsely chop parsley.  Toss these ingredients into a bowl along with the remaining 1/4 tsp of salt, 1/4 tsp pepper, olive oil and lemon juice, gently fold together.  

Toast hamburger buns a clean grilling area or skillet, serve hamburger on the bun, topped with lettuce and a few slices of avocado; salad on the side.