1 baguette, thinly sliced and toasted, for serving
In a food processor, combine the tuna with the mayonnaise, onion, cream cheese and lemon juice and pulse until smooth. Add the parsley and pulse just until combined. Transfer the mousse to a bowl, season with salt and pepper and refrigerate for at least 1 hour. Serve chilled, with the baguette toasts.
The mousse can be refrigerated for up to 8 hours.
Try an Italian sparkling wine to balance the salty, creamy mousse.