Preheat the oven to 415 degrees F. Place parchment paper on a large baking sheet and spray with cooking spray. Set aside.
Brush any dirt off of the portobellos, and remove the stem. Slice them about 1/6 inch thick and place them in a large bowl.
In a separate bowl, mix together the marinade. Add the maple syrup, liquid smoke, garlic powder, rice vinegar, salt, dijon mustard to the bowl and whisk until combined.
Pour the marinade over the mushrooms and toss until every mushroom is coated with the mixture. Do this step carefully, as the thin sliced mushrooms can break easily. Spread the mushrooms evenly over the parchment lined baking sheet so every mushroom is laying flat.
Bake the mushrooms for about 35-40 minutes. You will need to transfer the smaller bits to a plate before the bigger bits are done, or they will burn. You will also likely need to flip the mushrooms towards the end of cooking, around the 30 minute mark. Remove the pan from the oven and allow to rest. Enjoy!
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