Cook Brussels sprouts in a pot of lightly salted boiling water for 15 minutes, or until fork-tender; strain through a colander, removing as much water as possible. Set aside.

Melt butter in a large skillet over medium-high heat. Cook and stir mushrooms until lightly browned.

Toss Brussels sprouts with mushrooms and sprinkle with parsley, lemon juice, salt, and pepper. Serve immediately.