Melt butter with olive oil in a large sauté pan over medium heat. Add onion with a pinch of salt, cover, and cook until soft, about 5 minutes, stirring occasionally.
Increase heat to high, add mushrooms, and cook for 8 minutes until golden and liquid has evaporated.
Stir in garlic, thyme, and nutmeg; cook for 30 seconds.
Add Marsala (or white wine), cooking until mostly evaporated and mushrooms are glossy.
Stir in lemon juice, truffle sauce, salt, and pepper.
Transfer mixture to a food processor with cream cheese. Process until smooth, scraping down sides as needed.
Spoon into ramekins or jars, smooth the top, and refrigerate for at least 2 hours.
Optionally, top with melted butter, a mushroom slice, and a thyme sprig before chilling.
Serve with crusty bread or crackers.
