For the Sauce:

1. To make the sauce, heat a large fry pan over medium-high heat. Note: The large surface area of the pan will help the mushrooms get good color.
2. Once hot, add the shallots and cook for 30 seconds or so, then add the mushrooms and stir to combine. Let cook until the mushrooms start to release their juices, stirring frequently. Once the mushrooms start to release their juices, turn up the heat until all of their juices have evaporated and the mushrooms are nice and golden. Again, make sure to stir frequently to prevent them from burning. Next, deglaze the pan with the Madeira. Let the Madeira reduce down by about half and then add the stock. For the stock, we used this Mushroom Stock. Also note that if you do not want to use any cornstarch (or the Madiera even), the stock can be reduced down to a more sauce-like consistency before adding it to the dish.
3. Next, add the Dijon and horseradish and let the sauce cook for 10 to 15 minutes.
4. Meanwhile, go ahead and make the slurry — by combining together the cornstarch and water. *Note: You may need to use a bit more cornstarch (or arrowroot) to thicken the sauce; it really depends on how much your sauce has reduced and the final consistency you are looking for.
5. The tempeh can also be prepared while the sauce is cooking.
6. After the sauce has cooked for about 12-15 minutes, whisk in the slurry. Once you have reached the desired consistency, let the sauce simmer for a minute or so, to cook out any cornstarch flavor.
7. At this point, turn off the heat, add the fresh tarragon and taste for seasoning.

For the Tempeh:

1. Note: To save time, rather then braising the tempeh in this step, it can simply be fried and then placed into the sauce to braise.
2. If braising the tempeh first — which will add moisture and flavor to the tempeh — use a large pot, and combine together the tamari, garlic and water and bring to a simmer.
3. To prepare the tempeh pieces, cut each piece of tempeh into about 8 to 10 thin slices, diagonally. *Note: How the tempeh is ultimately sliced will depend on what shape the tempeh was to begin with. In the end, you just want even-sized pieces to work with. Add the tempeh to the pot and continue to gently simmer for at least 45 minutes.
4. Once the tempeh is ready, drain onto a tray lined with a cooling rack—or something that will allow the tempeh to dry as it cools.
5. Just before frying the tempeh, season it with salt and pepper. To fry the tempeh, heat a large fry pan over medium to medium-high heat. Once hot, add the oil, followed by the tempeh. Note: If using a non-stick pan, the tempeh can be fried using no oil, if desired.
6. Cook for 2 or 3 minutes on the first side, or until nice and golden. Then flip and cook for another 2 minutes or so. Remove from the pan and set aside while you prepare the sauce.