Slice portabellos into chunky strips. Heat oil and butter in a large frying pan over medium-high heat. Add portabellos and cook, tossing them often until just tender and golden brown.Add the miso paste and toss to coat them well. Remove from the heat and set aside. Spread a thin layer of mayonnaise onto two slices of bread. Top with lettuce and sliced tomato. Season the tomato. Top the tomato with miso basted portabello slices. In a small bowl roughly mash the avocado with a fork. Season with salt and lemon juice. Spread onto the other two slices of bread and top the sandwiches. Serve. *Sandwich tip: Wrap sarmies tightly in baking or wax paper. Tie with a string. Slice in half and then eat easily without losing any goodness!