In a large skillet, heat a drizzle of olive oil over medium heat. Add the onions, season with salt and pepper, and sauté for 1 to 2 minutes.
Reduce the heat to low. Add the thyme sprigs and bay leaves. Cover and cook for about 25 to 30 minutes, stirring occasionally, until the onions are very tender and lightly caramelized.
At the end of cooking, add the fig balsamic vinegar and continue cooking for another 2 to 3 minutes to allow the liquid to reduce slightly. Remove the laurel leaves and thyme sprigs. Stir in the old-fashioned mustard and the cream. Mix until the filling is well combined and smooth. Adjust seasoning if necessary. While the onions are cooking, roll out the dough on a lightly floured work surface. Cut out circles of dough and place them into greased tartlet molds. Prick the bottoms with a fork.
Bake the tartlet shells with a weight on top (such as a small ramekin) for about 15 minutes.
Distribute the caramelized onion mixture evenly among the tartlet shells. Top generously with grated Gruyère cheese.
Increase the oven temperature to 425°F (220°C) and continue baking for 18 to 20 minutes, until the cheese is thoroughly melted and golden brown.
Let rest for a few minutes before removing from the molds and serving.
