Preheat oven to 400 F. & place rack in the middle. Line a baking sheet with foil paper & set aside.
Brush the mushroom caps (top & bottom) with Italian dressing & place them, gills side up , on the lined baking sheet. Bake 10 minutes then flip them over & bake for an additional 10 minutes.
When mushrooms are ready & the juices have been released, remove them from the baking sheet. Place them on a wire to drip off a bit.
Guacamole
On a piece of plastic wrap, mash avocado with lime juice, salt, garlic, onion & cilantro. Fold plastic wrap over guacamole & set aside in fridge.
Beef Burgers
Preheat barbecue grill (or roast burgers in oven).
In a bowl, combine all burger ingredients & mix well. Divide beef mixture into 4 equal parts & shape into patties. Grill patties 3 minutes on each side or until desired degree of doneness. Divide cheese between patties & allow to melt.
Assembly
Top each of the 4 mushroom caps with some guacamole, a burger patty & tomato slices. These are definitely the kind of ‘burger’ you want to eat with a fork & knife. Of course you could always squeeze the whole thing in a ciabatta bun!