Use a towel to gently rub the portobello mushrooms to help remove any dirt, then arrange them in a glass baking side, with the stem-side facing up.
In a small bowl, stir together the tamari, vinegar, garlic, sage, and thyme. Use a spoon to pour the mixture over the 3 mushrooms, then use your hands to rub the sauce on the outside of the mushrooms, too. (Alternatively, you could put the mushrooms in a sealed container and gently shake them to coat them in the marinade, too.)
Let the mushrooms marinate for 15 minutes, while your oven preheats to 400ºF.
Bake the marinated mushrooms for 15 minutes, then use a spatula to flip them over and bake for an additional 15 minutes. It doesn't matter which side you bake first, but I think they look prettier for serving when you start with the stem-side up, then flip them over so the mushroom cap is nice and golden.
Serve warm with your favorite side dishes and an extra sprinkling of fresh sage or thyme for presentation. Leftover mushrooms can be stored in an airtight container in the fridge for up to 3 days.