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Drizzle the mushrooms and the onions with olive oil.
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Place mushrooms on foil and grill until tender, about 10 minutes.
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Place onion slices directly on grill and cook, flipping halfway through, about 10 minutes.
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Place turkey burgers on grill, baste with barbecue sauce, and cook about 4-5 minutes per side. Add more sauce after you flip them.
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Add a slice of gruyere cheese to one patty and top with another patty.
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Lightly toast the buns on the grill.
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Add barbecue sauce to the bottom bun. Stack the onion, burgers, mushrooms, and watercress on top.