Slice the tempeh in half so you have two square-ish pieces. Then carefully slice each of those squares in half through the thinnest side, kind of like slicing a roll or bagel.
Heat a large pan over medium high heat and add 1 teaspoon of the oil along with the tempeh slices. When the tempeh beings to brown, flip the slices. It will be ready when it’s golden brown and a little crispy on both sides. If the tempeh sticks to the pan, you can add a little vegetable stock or water.
While the tempeh is cooking, cook the mushrooms. Add the remaining oil to another large pan over medium-high heat. Cook the onions for several minutes, until they become translucent start to brown slightly. Add the garlic, mushrooms, salt, and thyme and continue cooking until the mushrooms have started to brown, about 5 minutes.
Whisk the cornstarch in the stock and add it to the pan along with the wine. Bring to a boil and then reduce heat. Let it simmer for 10 minutes or until the liquid has thickened and reduced by about half.
Serve the tempeh marsala by pouring the sause over the tempeh cuts, and serve on top of the cooked brown rice, mashed potatoes, or pasta, if using. Garnish with parsley or chives.