To begin, make sure your mushrooms are clean. If they have already been washed, you can use a small brush to just run over the mushrooms and ensure that they are clean and free of any dirt or soil.
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Now, get a shallow pan, and line all the mushrooms caps up in this, with the smooth side up. Take a small bowl, and add in the balsamic vinegar, the olive oil, dried basil, dried oregano, minced garlic, and salt and pepper to taste. Whisk them all together until they are thoroughly combined.
Pour this marinade over the mushrooms, making sure they are all well coated with it. Allow them to soak up the flavours for around 10 minutes, at room temperature. You can turn them twice during the course of this, to make sure they are evenly coated with the marinade.
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While the mushrooms are marinating, preheat your grill on medium-high heat, and use a brush or a cloth to oil the grates. Once the 10 minutes have elapsed, line your Portobello mushrooms up on the grates, and grill them for 5 to 8 minutes on each side, basting frequently with the leftover marinade.
During the last 2 minutes of grilling, top the mushrooms with a slice of provolone cheese each. Once it is melted, your grilled Portobello mushrooms are ready. To serve, slice a whole wheat bun, spread it with your preferred sauces, layer on the Portobello mushroom, along with a slice of tomato, grilled onions, pickles, and crisp lettuce leaves, and top with the other half of the bun. Serve with healthy baked sweet potato fries.
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