Peel and finely chop the onion. Halve the fennel bulb. Slice off the dry tips and set aside any soft fronds for later. Slice it into 1cm thick fingers, then finely chop it. Put a large pan over a medium heat and let it warm for 1 min, then add the chopped veg with 1 tbsp oil and a pinch of salt and pepper. Gently cook for 5 mins, stirring occasionally, till the veg have softened a little.
While the veg sweat, finely chop the tomatoes. Peel and grate or crush the garlic. Finely chop the coriander stalks, saving the leaves for later. Halve the lemon and slice one half in half again to make wedges.
Stir the tomatoes into the veg and let it cook down for 8 mins, stirring occasionally, till the tomato is soft and pulpy and has a rich red colour.
Add the garlic and coriander stalks to the pan. Measure out 1 tsp turmeric and 1/2 tsp each ground ginger and cumin and add them with the cinnamon stick. Squeeze the juice from the lemon quarters into the pan and the drop them in. Give everything a good stir to mix.
Add the olives and crumble in the stock cube. Pour in 400ml boiling water and stir to mix well. Pop a lid on the pan and simmer for 5 mins to mingle all the flavours.
While the tagine stock simmers, tip the couscous into a heatproof bowl. Pour in 300ml boiling water. Pop a plate over the bowl to cover it (or use cling film) and leave it to stand for 5 mins. The couscous will soak up the water.
Carefully add the fish pie mix to the pan, then pop the lid back on and simmer for 5 mins till the fish is cooked. The white fish will be opaque and any salmon will be a dark pink. They will flake easily if you press them with a fork.
Taste the tagine and add a pinch more salt and pepper if you think it needs it. Fluff the couscous up with a fork and spoon it into two warm bowls. Ladle the tagine over the top, discarding the lemon wedges and cinnamon stick. Garnish the tagine with the coriander leaves and fennel fronds and serve with the remaining lemon wedges for squeezing.