Using a large bowl, combine the maple syrup and liquid smoke and stir well. Rinse or wipe the portobello mushrooms clean. Slice the portobellos into 1/4 inch thick pieces and place in bowl to marinate for 20 minutes.
Finely mince the garlic. In a small bowl, combine the garlic and mayonnaise. Season with a pinch salt.
If desired, toast whole wheat bread.
Place a nonstick skillet over medium high heat and add 2 TBSP vegetable broth. Once hot, add the mushrooms, seasoning with salt and cook until they start to brown, 2-3 minutes per side. Once finished, remove and place on paper towel to slightly cool
To complete your sandwich, place a layer of mushrooms on one piece of the whole wheat bread. Add sliced tomato and the lettuce. Spread the garlic mayo on a 2nd piece of bread and place on top.
For ease of eating, cut your sandwich in half and enjoy!