Season the rabbit pieces with salt and a light coating of Dijon mustard, then set aside to marinate for 20 minutes.
In a pot, warm some olive oil or butter over medium heat. Add the onions and cook until soft and translucent. If desired, add garlic and sauté briefly for added flavor.
Place the rabbit pieces in the pot and brown them evenly on all sides.
Pour in broth or white wine to deglaze the pot, stirring to lift any browned bits from the bottom. Cover the pot and let it simmer for about 15 minutes.
Mix in the cream and additional Dijon mustard, adjusting the seasoning with salt and pepper as needed. Cover and cook for another 15 minutes, allowing the rabbit to become tender and the sauce to thicken.
For an optional touch, transfer the pot to a preheated oven at 375°F (190°C) for 10 minutes to achieve a golden caramelized finish.
Arrange the rabbit on a serving dish, spoon the creamy mustard sauce over the top, and serve with your preferred sides.