First you'll want to marinate and cook up your portobello mushrooms.
Wash mushrooms.
Remove stems, and scrape out gills with a spoon if desired (I did here).
Mix together 1/4 C balsamic vinegar, 2 Tbsp olive oil, and garlic, then pour over mushrooms, add a little bit of ground pepper and sea salt, and let marinate in nonreactive dish for about an hour, flipping occasionally.
Place marinated mushrooms on foil-lined cookie sheet and cook for 10 minutes at 400, then flip and cook another 10 minutes or until done.
Slice your cooked mushrooms into even pieces, and they're ready to go in your panini. So, let's get going with that...
Brush one side of a piece of sourdough bread with olive oil, then place oil side down on grill pan.
Spread lightly with Dijon mustard.
Add two pieces of provolone and one sliced cooked portobello mushroom.
Add some marinated artichoke, baby spinach, and roasted red pepper.
Then, top with a second slice of sourdough bread brushed with olive oil.
Now comes the "press" part, lol. Here, I used a second pan and weighted it down with my foil-wrapped brick to press my sandwich nicely. (You don't have to use a brick; just use heavy cans or whatever you have handy to weigh down your second pan.)
Cook for three minutes on medium-high heat, then flip over and repeat for another three minutes. Be careful, because the fillings will try to spill out (and this is where a panini press would come in handy).