Remove stems so the mushrooms lie flat. Place each mushroom in the marinade for 3-5 minutes per side.
Preheat grill to medium heat while mushrooms are soaking.
Place mushrooms on grill top side down and cook until darkened, about 5 minutes. Flip and cook the other side until golden.
Remove from the pan and place on a rimmed plate to cool. Brush the tops with a little more of the marinade.
Once cool enough to handle, slice into strips.
In a large bowl, tear lettuce into bite size pieces.
Slice the cherry tomatoes into halves and toss with the lettuce.
Divide the salad among four serving bowls and top each with the avocado and goat cheese.
Add the grilled portobello mushroom strips and serve immediately with the lemon dressing (stir again to blend before adding).