In a large skillet, melt the butter with the lemon juice over medium heat. Add the mushroom caps, cover, and cook, for 5 minutes. Add the olive oil, increase the heat, and cook, stirring, for 2 minutes. Lower the heat and simmer, stirring occasionally, until soft, 20 minutes. Add the shallots, garlic, salt, and pepper, and cook, stirring, for 3 minutes.
Remove from the heat and adjust the seasoning, to taste. Sprinkle with the parsley and serve.