Marinate the Portobello well ahead of time for ultimate depth of flavour, and then throw them on a pan or barbecue for a quick and delicious vegetarian burger!
mix all ingredients except the mushrooms together in a container or resealable bag
wipe off the portobello caps as best as you can
toss them into the marinade and shake em up so they're well coated, then throw them in the fridge
i only did this a few hours before cooking them and they still managed to absorb a lot of delicious rosemary flavour! i bet if you did this the day before, or the morning before, you'd get even more.
this will be a messy burg! especially if you're frying them in a pan, be prepared for a sloppy joe-like experience. barbecue might result in a more dry burger. let me know!
you want them to be cooked through and tender, and the balsamic will sort of caramelize in the pan.
rough chop the scapes and mix in with the mayo and lemon - easy peasy - we only made a very small serving because we are only two people and one of us is taking it easy on the garlic, but that's the basic ratio that works for a very flavourful mayo.
for this recipe, which i found on Spruce Eats, they recommended using a blender. that would certainly speed things up, but also mess up another kitchen accoutrement. i just throw all the ingredients in a mason jar and shake.