Heat 2 tablespoons of water in the base of a medium pan. Add the chopped onion and celery/broccoli stem and bay with a pinch of salt. Sauté on a medium heat for 5 minutes, stirring from time to time.
Add the sweet potato and the tamari and continue to sauté on a low heat until the potato is just starting to soften.
Whilst the veggies are cooking, split the shiitake mushrooms into two piles. Slice half and chopped the other half.
Add the sliced mushrooms to a small pan with 1 tablespoon of water and lightly 'fry' for a few minutes - this is particularly important if you are using white closed cup mushrooms as they tend to leak a lot of fluid when cooked - you need to cook this away or it will make the centre of your loaf soggy. Shiitake mushrooms don't do this so much so you can even skip this step if you wish.
Add the chopped mushrooms and garlic to the pan and cook for another 3 minutes. Then turn off the heat.
Pre-heat the oven to 180ºC/350ºF/Gas4. Line a 2lb loaf tin with non-stick baking paper.
Tip the veggie mix into a large bowl. Add the cooked lentils, herbs, oats and ground flaxseed. Season well with salt and pepper and mix together well.
Spoon half the mix into the prepared tin and press it into the base and corners. Spread the sliced mushrooms over the top then cover with the remaining mix. Press down and flatten the top.
Bake in the oven for up to 30 minutes until the top is lightly browned and firm to touch. Remove from the oven and leave to settle in the tin for 10 minutes before slicing and serving.