Preheat the oven to 180 degrees C/350 F and line a large baking tray with parchment paper.
Prepare the potatoes and eggplant by slicing them into thin circles, then transfer to a large mixing bowl. Drizzle with olive oil, and stir together well.
Arrange the vegetables on the baking tray, and bake for 25 minutes, until crispy and fully cooked through.
Make the lentil filling
Cook the red lentils for 8-10 minutes, until softened but not mushy. Drain and rinse when done.
Heat olive oil in a large frying pan or saucepan. Add the onion, garlic, carrots, bell peppers, and tomato paste. Cook for 5 minutes, until softened.
Now add the fresh tomatoes, mushrooms, paprika, dried oregano, and tamari. Cook for a further 5 minutes, stirring frequently.
At the last minute, stir in the cooked lentils together with the lemon juice.
Make the vegan Bechamel
Melt the vegan butter in a saucepan, then add the flour and ¼ cup of the plant based milk. Whisk together and once no lumps remain, gradually add the rest of the milk and whisk continuously. The Bechamel is done when it's very thick and smooth. The whole process should take about 5 minutes.
Assemble and bake
Add a layer of the roasted potatoes and eggplant circles to the bottom of a large rectangular dish. Next, add the lentil filling and flatten it down gently. Follow this up with the rest of the vegetables and the Bechamel sauce.
Cover the baking dish with foil and bake in the preheated oven for 20 minutes. Then, remove the foil and bake for a further 5 minutes, until lightly golden on top.
Tips and substitutions
In order to keep this vegan Moussaka gluten-free, simply use tamari as opposed to soy sauce, and use gluten-free flour for the Bechamel sauce.
You can also fry the vegetables in a frying pan with a little bit of olive oil as opposed to baking them.
Use canned lentils if you want to save time/washing up.
Make sure to whisk the Bechamel continuously, and don't walk away from the stove, as this may cause it to become lumpy.
Instead of the Bechamel, you can use mashed potatoes or mashed cauliflower.
Instructions for storage and freezing
You can keep this Moussaka in the fridge for up to 4 days in an airtight container, and reheat in the microwave in 60 second intervals, until hot.
You can also freeze it for up to 2-3 months. Reheat from frozen in an ovenproof dish for 40 minutes at 180 degrees C/350 F.