Gently heat 1 tablespoon of the oil in the tagine, and fry the onions for a few minutes until they become translucent. Add the smashed garlic, and continue to cook for another 2-3 minutes, until the raw garlic smell has gone.
Turn off the heat while you layer the vegetables: the ones which take the longest to cook should be at the bottom, while the ones which require the least time should be on the top. I do it in this order: potatoes (which I stir in with the onions to coat them in oil), then half the preserved lemon, carrot, half the coriander and parsley, pumpkin, half of the parsley, aubergine, courgette, French beans, the rest of the parsley, and the tomato. You should have a dome of food now!
Push the olives in anywhere you can - you don't have to be precise - and sprinkle a few on the top.
If using, arrange slices of seitan around the edge of the dome.
Whisk the Ras el Hanout and the remaining oil into the hot vegetable broth, until well-blended (it will thicken). Carefully pour all over the food.
Put on the tagine lid on, and turn the heat onto its lowest setting. If you're using gas, then a diffuser will come in handy.
The dish should take around 40-50 minutes to cook. It's possible the juices may overflow a little but don't worry - just clean it up afterward!
Carefully remove the tagine lid to check that the veggies are soft, then replace the lid, turn off the heat, and leave the tagine to stand for five to 10 minutes.
Bring to the table, sprinkle the rest of the coriander over the top, and dig in. Serve with fresh bread or couscous (or both).
Or use 1 vegetable stock cube (low sodium if possible) dissolved in 180ml (¾ cup) hot water.