Prepare the Rabbit: Rinse the rabbit pieces under cold water and pat dry with paper towels. Season with salt and pepper.
Sear the Rabbit: In a large skillet or Dutch oven, heat the olive oil over medium-high heat. Add the rabbit pieces and sear until they are golden brown on all sides, about 5-7 minutes. Remove the rabbit from the skillet and set aside.
Sauté Aromatics: In the same skillet, add the chopped onion and garlic. Sauté until the onion becomes translucent, about 3-4 minutes.
Make the Sauce: Pour in the chicken stock, scraping any browned bits from the bottom of the pan. Stir in the heavy cream, Dijon mustard, and whole grain mustard. Bring the mixture to a gentle simmer.
Combine and Cook: Return the rabbit pieces to the skillet, ensuring they're coated in the sauce. Cover and let it simmer on low heat for about 30-40 minutes, or until the rabbit is tender and cooked through.
Serve: Taste the sauce and adjust the seasoning with more salt or pepper as needed. Serve the rabbit hot, garnished with fresh thyme or parsley, alongside crusty bread or mashed potatoes to soak up the delicious mustard sauce.