Slice the tempeh about 1/4-inch thick. In a flat, microwavable baking dish arrange the tempeh slices in a single layer. Mix the remaining ingredients and pour over the tempeh.
In the microwave, cook on high power for 3 minutes. Turn or rearrange any pieces that were not covered by broth and microwave on high for 2 minutes. Then cook at 50% power for 3 more minutes. Allow the tempeh to stay in the broth until you’re ready to assemble the casserole.
Make the sauce by putting all the sauce ingredients except the tarragon into a blender and blending until completely pureed. Stir in the tarragon just before using the sauce.
Preheat the oven to 400F. Oil a large, 3-quart casserole (or use two smaller casseroles). Wash the potatoes and cut them in half lengthwise. Slice each half into 1/4-inch slices.
Put half of the potato slices in the bottom of the casserole dish. Lift the tempeh out of its marinade and arrange it on top. Put the remaining potatoes over the tempeh. Stir the sauce and pour it over the potatoes. Sprinkle with freshly ground black pepper and cover tightly.
Bake for 45 minutes. Remove the cover, sprinkle with vegan Parmesan, and bake for 10-15 more minutes, until potatoes are tender and sauce has thickened. Remove from the oven and let stand a few minutes before serving.