In a medium mixing bowl, combine the softened goat cheese and mascarpone. Use an electric hand mixer on medium speed to beat them together until completely smooth and unified, about 1–2 minutes.
Add the crème fraîche, chopped thyme, and black pepper. Continue beating for another 30–60 seconds. The mixture should be thick, smooth, and hold a firm peak.
Transfer the mousse to a piping bag fitted with a star or plain tip. Let it chill in the refrigerator for at least 30 minutes before using.
Pipe a generous dollop onto each cracker or toasted crostini. Top with a small spoonful of your onion balsamic jam.