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Start by trimming the ends of the onions and peeling them. Use a mandolin to slice the onions thinly.
In a large bowl, combine the sliced onions with finely chopped thyme, a generous amount of salt, and black pepper. Mix everything together using your hands.
In a small pot, mix the beef better than bouillon with heavy cream and bring it to a boil, stirring well.
Once boiling, remove the pot from heat and stir in a generous spoonful of Dijon mustard into the cream mixture.
After letting the onions sit for about 10 minutes, use a paper towel to blot away any excess moisture.
Begin layering the onions in a baking dish, starting with a layer of onions, followed by a layer of the cream mixture, and then another layer of onions.
Finish with a layer of Gruyère cheese on top and dot with butter.
Cover the dish and bake it in a preheated oven at 325 degrees for approximately 45 minutes.
After 45 minutes, remove the cover and continue baking at 325 degrees for an additional 30 minutes.
Allow the dish to rest for about five minutes before serving.