Sauté the onions with the olive oil over medium heat for several minutes until golden and caramelized.
Remove and discard the stems from the mushrooms. Slice the caps vertically into ½" strips. Spray with pan spray, season with salt and pepper, and then grill
until moisture is mostly rendered and the slices are darkened.
Stack the slices onto the bottoms of the buns. Top with cheese and onions, then warm on a sheet pan at 400° in an oven until the cheese begins to melt.
Toast the tops along side.
Remove from the oven, top the onions with tomato slices, slather the bun tops with BBQ sauce, and assemble.
Serve with oven fries and a fruit salad.
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