• Preheat oven to 400 and line a large rimmed baking sheet with unbleached parchment paper. OPTIONAL: Place a wire rack on top of the parchment so mushrooms will cook, without sitting in their natural juices that will release during cooking.
• Gently clean any dirt from mushrooms with a damp paper towel. DO NOT immerse mushrooms in water as they will soak it up like a sponge, creating an unappealing texture.
• Trim the bottom, woody part of the mushroom stems, creating a flat surface for them to stand on while cooking. Lay them onto the prepared baking sheet, spaced out.
• Use a pastry brush to evenly apply the thinnest layer of oil to stem side of mushroom, sprinkle with salt and pepper. Flip onto other side and repeat, brushing caps with remaining oil, then sprinkle with salt and pepper.
• Roast in preheated oven for 10 minutes. Remove. Flip to other side and return to oven for additional 10 minutes.
• While the onions are cooking in the oven, make the chimichurri.

Chimichurri

• Remove leaves from stems of the fresh herbs. I used 1 cup cilantro, ¾ cup parsley and ¼ cup oregano. Use whatever proportion you have to equal 2 cups. Chop finely and add to medium sized mixing bowl.
• Finely chop shallot, add to bowl. Use a microplane to grate the garlic right over the bowl, and the same for the outside of the washed lime. Then squeeze the lime juice into the bowl, add the vinegar and olive oil too. Sprinkle in the salt and crushed pepper and mix well.

Finish and Serve

• When mushrooms have finished cooking, flip them over to pour out any remaining juice onto the parchment. Then transfer the cooked mushrooms to a bowl.
• Spoon on chimichurri and serve the remaining sauce on the side in a small dish.
• OPTIONAL: Depending on how large your portobello mushrooms are, you may want to slice them. Use a steak knife and cut them into slices, toss well in the sauce, adding more chimichurri as desired and serve like a mushroom salad.