Place a rack in the lower third of the oven and preheat the oven to 350 degrees F.
If you are using whole rabbits, place a rabbit flat on a cutting board. Separate the two back legs. Cut the 2 front legs from the shoulders. Cut the back loin into 3 pieces cut across the ribs, then the rack into 3 pieces. Cut each of the back legs in half. You should have 12 to 14 pieces. Repeat with the remaining rabbit.
Bring the stock and wine to a boil in a large saucepan. Reduce the heat to medium-low and keep at a slow, steady simmer.
Warm the olive oil and butter in a 7 quart cast iron low-sided braiser over medium-high heat. Season the rabbit with salt and pepper and dust with the flour. Add the rabbit and sear until golden brown on all sides, about 10 minutes. Add the onions and mushrooms and sweat, while stirring, for 5 to 7 minutes. Add the mustard seeds and the whole tarragon leaves, toss well and bake for 10 minutes. Pour in the hot stock/wine mixture, cover with a round of buttered parchment paper and bake for 30 minutes. Remove and discard the parchment paper and bake 10 minutes more.
Meanwhile, whisk together 3 tablespoons of Dijon mustard, the mustard powder, and the remaining chopped tarragon leaves. Brush the rabbit with the mustard mixture and bake for 15 minutes. To check for doneness, using a small knife pierce the thickest part of the thigh. The meat should feel tender. Stir in the remaining 1 tablespoon of mustard and sprinkle the croutons and chives over. Serve.