Thoroughly wash mushrooms. Using a chef’s knife, cut off any long stems so the mushroom is approximately even height on the under-side.
Place mushroom between two tea towels and put a heavy object that covers the mushrooms on top (such as a thick wooden cutting board) to help squeeze out excess water.
Meanwhile, tear off 4 large (approximately Portobello sized) lettuce pieces. Rinse well before setting aside on a third tea towel to dry.
Once dried, lightly coat both sides of each mushroom with oil to prevent sticking to the grill.