On the topic of a perfect burger...
One thing to always consider if you want to add a little spice and savory taste to any hamburger is to consider shipping in some New Mexican green chile peppers, roasted, skinned, and seeded. Green chile can often be found frozen, but this little known ingredient is a great way to add to a portabella melt, maybe with the provolone cheese, or maybe diced and dashed within tomato slices. You can even blend it in with a homemade mayonnaise. It isn't a super spicy pepper (like Tai green peppers) but is more like a slightly warm spicy bell pepper.
If anyone else in the forum knows about this ingredient, they will understand where I am coming from since it is truly a game changer for many recipes, especially hamburgers.
Just something I thought you would find very interesting.
Enjoy the very best,