Preheat the oven to 400 degrees Fahrenheit. Line 2 baking sheets with parchment paper. (See notes)
In a large bowl, toss the sliced mushrooms and onions with salt and pepper. Spread the veggies in an even layer on two baking sheets, and bake for 25 minutes, or until the mushrooms and onions are golden brown. Turn over after 15 minutes.
While the mushroom and onions are roasting, make your BBQ sauce (if using homemade) and your slaw.
To make the purple cabbage slaw: toss everything together in a medium mixing bowl, and allow to marinate for at least 15 minutes or more. Toss every 5 minutes or so.
Place the roasted mushrooms and onions in a medium mixing bowl, and toss with 3/4 cup (or more!) of the BBQ sauce. Heat gently in a sauce pan over medium heat.
Place 1/4 of the BBQ mushrooms and onions on each bun, and serve with pickled purple cabbage slaw as a topping or on the side.
• If you don't care for mushrooms, you can substitute about 2 cups of cooked lentils. Just cook your onions up in a skillet, then add your cooked lentils and BBQ sauce when the onions are ready. • You can use white mushrooms, creminis (also called baby bellas), shitakes--or pretty much any mushroom you like for this recipe. To save time, you can usually find white mushrooms already sliced.• For a creamy slaw, drain the excess vinegar from the slaw before adding 1/4 cup (or more) of mayo.• If you don't want to heat up your oven, you can saute your mushrooms and onions in a skillet until they're golden brown. Just use water a tablespoon at a time to keep things from sticking.