Chop the portobello mushrooms into 1cm-thick slices. Peel and finely chop the onion. Trim and cut the celery stick into pieces to match the onion. Trim, peel and finely chop the carrots.
Place a large pan or wok on a medium heat for 2 mins, then drizzle in 1 tbsp olive oil. Add the mushrooms and fry, stirring, for 3 mins till golden and slightly softened. Lift them out of the pan and on to a plate with a slotted spoon.
Pour another 1 tbsp olive oil into the pan. Add in the onion, celery and carrot with a pinch of salt. Fry for 6 mins on a medium heat, stirring occasionally. The veg should be softened but not coloured.
While the veg fry, peel and crush the garlic cloves. Halve the pepper and scoop out the seeds. Chop into 2cm pieces. Halve the jalapeño chilli, if you’re using it, and flick out the seeds and white pith Finely chop half or the whole chilli (jalapeño chillies are hot, so if you just want a little spice just use half the chilli).
Add the garlic, pepper and chilli (if using it) to the pan and fry, stirring, for 2 mins. Tip the portobello mushrooms back into the pan along with 1 tbsp smoked paprika, and stir.
Tip in the tin of cherry tomatoes and pour in 600ml boiling water. Bring to a bubble then tip in the rice. Stir together. Pop on a lid and simmer on a low heat for 25-30 mins till the water has been absorbed and the rice is al dente (soft with a slight bite). Stir every now and again. Add a splash more water if the rice is sticking.
While the jambalaya cooks, zest the lemon then slice it into wedges. Finely chop the coriander stalks and roughly chop the leaves.
After 25-30 mins the rice should be cooked. Taste the jambalaya and adjust the seasoning, adding a pinch of salt or pepper. Stir through the finely chopped coriander stalks and half of the leaves.
Spoon the portobello jambalaya into warm bowls. Scatter over the lemon zest and remaining coriander leaves. Serve with the lemon wedges to squeeze over.