Preheat oven to 425 degrees Fahrenheit. Line and grease a baking sheet.

Add portobello mushrooms and haloumi slices to the baking sheet. Brush mushrooms with barbecue sauce and sprinkle with salt and steak seasoning. Spray everything with nonstick cooking spray. Bake for 15 minutes, until haloumi is golden and mushrooms are cooked through.

Turn oven to broil. Add tomatoes and onions to a second baking sheet. Spray with nonstick cooking spray and sprinkle with salt (add steak seasoning here as well if you like your black pepper!). Broil until tomatoes are soft and both tomatoes and onions are starting to get charred at the edges (this can take 8-10 minutes, but watch it carefully, as cooking time will vary depending on heat of your broiler and how close your baking sheet is to the heat source).

Assemble your burgers: Slice open an onion bun and layer onions and tomatoes on the bottom. Top with cheese and portobello mushrooms. Drizzle with a bit more barbecue sauce and enjoy.