Preheat oven to 425ºF with a roasting pan, preferably cast iron, on a rack in a lower position. Toss mushrooms, onion slices, and garlic together and sprinkle with thyme and cayenne peppers. Drizzle with melted butter and soy sauce and toss; spread in an even layer on preheated roasting pan and cook, stirring occasionally, until browned and softened, 10 to 12 minutes. Transfer to a serving platter, toss with lemon juice, taste, and adjust seasonings.