Wipe the mushrooms clean with a damp paper towel and cut into 1/2 -inch slices.
In a large skillet over medium-high heat, pour in roughly a teaspoon of olive oil and sauté sliced portobello mushrooms. Cover, so the heat can distribute more evenly across the mushrooms. You may need to drizzle some more olive oil, if your mushrooms seem dry after a couple of minutes in the pan. Check occasionally and stir to allow all the mushrooms to have shared contact with the pan.
Once all the mushrooms cook down and look moist, season with salt and pepper. Drizzle 5-6 rounds of balsamic vinegar to lightly coat all the mushrooms. Please adjust the amount of vinegar to your liking. (I don't usually measure the vinegar - I just pour straight out of the bottle, so 5-6 rounds equates to about 2 tsp.). Stir to coat all surfaces of the mushrooms and cover for a minute to let all the flavors come together.
If you are using an aged balsamic, you don't need to cook the mushrooms for too long because it is already fairly thick. But if you are using balsamic vinegar that is thin in consistency, then you will want to leave the pan uncovered to allow the liquids to evaporate and thicken. After all, you want these mushrooms to have a nice glaze on them.
While the mushrooms are cooking, I like to prepare the pesto sauce. You can just use pesto or you can add some mayo (I like Vegenaise, which is vegan and has a nice creaminess). I just take a generous spoonful of Vegenaise and a generous spoonful of pesto and mix until blended.
Once you feel your mushrooms are done, turn off the heat and let them hang out to let the flavors come together. During this time, you can slice your ciabatta rolls and start toasting them in the toaster oven or oven at 350 degrees F for a few minutes (stick around closely so you don't burn your bread!).
Spread your pesto sauce (with or without mayo) onto each side of the bread, pile in slices of glazed mushrooms to cover the bread, top with fresh arugula and enjoy! Here, you have the option to get creative and add other toppings like melted cheese (muenster or harvarti would be nice), goat cheese, tomatoes, roasted red peppers, and/or avocado!