Place the portabellas into a large bowl and drizzle with balsamic vinaigrette, letting the mushrooms marinate for 30 minutes.
Remove the mushrooms from the bowl and season both sides. Heat a grill pan over a medium heat and warm some oil in the base.
Place the portabella's gill side down on the pan and cook for 4 minutes (or pop the mushrooms on the barbecue grill).
Turn and continue to grill the mushrooms are almost cooked through, about 4 minutes more.
Remove the mushrooms from the pan and let them drain on a paper towel (gill side down).
Cut each mushroom into quarters and place into the burger buns with the slices of onion, tomato and the lettuce leaves.
Cook's tip: If serving at a party as sliders (mini burgers), try securing all the fillings in place with cocktail sticks, then plates can be passed around without fear.