To make ratatouille with tempeh, you'll first need to prepare the ingredients. Heat the oil in a large skillet over medium-high heat. Add the onion and cook until softened, about 5 minutes. Stir in the garlic, thyme, salt, and pepper. Cook, stirring, until the onions are translucent, 2 to 3 minutes, then add the carrots, celery, green pepper and garlic. Continue to cook and stir for 1 minute. Remove the vegetables from the heat and set aside.