Cut the tempeh into slices or half slices about ¼ inch (6mm) inch thick and toss them in the cornstarch to coat evenly.
Heat up 2 tablespoons vegetable oil in a frying pan over medium-high heat. Fry tempeh until brown and a little crispy, 2 - 3 minutes on each side. Season with some freshly ground black pepper about halfway through.Add more vegetable oil if necessary.
Remove the fried tempeh from the pan and set aside. Let the pan cool down a bit to avoid overcooking the aromatics.
Make the sauce
Add an extra tablespoon oil to the frying pan over medium heat. Add red pepper flakes, crushed coriander seeds, minced garlic and grated ginger. Stir and fry for 1 minute.
Make a cornstarch slurry by stirring together the cornstarch for the sauce with an equal amount of water.
Combine soy sauce, water, and lemon juice. Add the liquids to the pan along with the brown sugar. Bring to a simmer and stir regularly until the sugar melts.
Add the cornstarch slurry while stirring, and simmer until the sauce thickens.
Place the fried tempeh back in the pan and coat it in the sticky sauce.
Serve topped with toasted sesame seeds and a sprinkle of sliced scallions, with a side of veggies and rice.