Tear the lettuce into bite-size pieces, rinse thoroughly and dry in a salad spinner.
Saute the mushrooms
Over medium heat, melt the two tablespoons butter in a large frying pan. Tilt the pan around to cover the entire bottom.
Spread the mushrooms slices evenly in the pan. Sprinkle with salt and pepper. Leave for about two minutes to allow the undersides to brown a little then cook, stirring often, for another two to three minutes until lightly browned.
Scoop out the mushrooms and transfer to a bowl.
Make the dressing
Peel and grate the garlic.
Add the garlic to the pan drippings and cook over medium-low heat for about 10 seconds.
Off the heat, add the rest of the butter. Swirl the pan until the butter is just melted.
Stir in the lemon juice.
Taste. Add more salt, pepper or lemon juice, or all of them.
Assemble the salad
Pour two tablespoons of dressing into a mixing bowl. Tilt the bowl to allow the dressing to spread to the sides.
Dump in the lettuce. Toss well.
Divide the lettuce among four salad bowls. Top with onion slices and the sauteed mushrooms. Drizzle in the remaining dressing. Serve at once.