Preheat the oven to 450° F. Line a baking sheet with aluminum foil.
In a small bowl mix the date syrup, maple syrup, and liquid smoke.
Remove the stems from the portobello mushrooms. Place the mushrooms on the lined baking sheet. Brush half of the syrup on the top and bottom of each mushroom. Bake for 5 minutes.
Remove from the oven and brush the other half of the sauce on the mushrooms. Bake 5 minutes more.
Remove the mushrooms from the oven and assemble burgers with your desired toppings. Enjoy!