Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and finely chop the onion. Place a large pan over a medium heat. Add 1 tbsp oil and the onion. Season. Cook and stir for 5 mins till the onion is soft and glossy looking. While the onion fries, finely chop the olives. Coarsely grate the cheddar.
Stir in the olives and add a pinch of chilli flakes. They're spicy, so add as much or as little as you prefer. Carefully pour in 450ml boiling water (the pan will splutter, so be careful you don't get splashed). Pop a lid on the pan, turn up the heat and bring the water back to the boil.
When the water is boiling, turn the heat down and very slowly trickle in the polenta, whisking as you add it to smoothly combine it with the water – be careful, the polenta can splatter when it hits the water, so add it gradually. Put a lid on the pan, turn the heat down to low and cook for 5 mins to thicken up the polenta.
Take the polenta off the heat and add most of the cheddar, keeping a little back for later. Taste and add a pinch more salt, pepper or chilli flakes if you think it needs it.
Slice the top 2cm off the peppers to create two lids and two pots. Scoop out the seeds and white pith. Place the peppers on a baking tray. Spoon the polenta into the pepper halves to fill them. Pop the 'lids' on the tray beside them. If you have any leftover polenta, keep it in the pan with the lid on, for later. Slide the baking tray into the oven and bake for 20 mins till the peppers are tender and the polenta is firm looking.
While the stuffed peppers cook, put a dry frying pan on a medium heat and add 1 tsp cumin seeds. Toast for 2-3 mins, shaking the pan occasionally, till the seeds smell nutty and start to pop. Tip them into a small bowl. Peel and crush or grate the garlic and add it to the bowl. Juice half the lemon into the bowl. Add 1 tbsp olive oil and a pinch of salt and pepper. Whisk to make a dressing. Set aside.
Trim the root and top 1cm off the spring onion. Finely slice it on the diagonal. Pop it in a large bowl. Halve the avocado, scoop the flesh out of the skins and discard the stone. Roughly chop the avocado flesh and add it to the spring onion with the salad leaves. Pour in the garlic and cumin dressing and toss to mix.
After 20 mins, take the peppers out of the oven and sprinkle over the last of the cheddar. Return to the oven for 10 mins till the cheese is golden and melted. Serve the stuffed peppers with the avocado salad on the side..