Peel and halve the red onion. Slice into thin wedges. Pop in a heatproof bowl. Pour over enough boiling water to cover. Set aside to soak.
Put a pan of salted water on to boil. Trim the ends off the runner beans and if there is a string, gently pull it off. Slice the runner beans on the diagonal into chunks around 1cm thick.
When the water is boiling, add the runner beans. Simmer for 2 mins, then drain and rinse under cold water to stop them cooking. Leave them in the colander to drain while you prepare the rest of the salad.
Pull the silks off the corn cob. Pull off any loose threads. Slice a slither off the bottom of the corn cob so you can sit it upright on the board. Run a sharp knife down the cob to slice off the corn kernels. Tip them into a large bowl.
Halve the avocado. Scoop out the stone. Scoop the flesh out of the skins with a spoon. Dice it. Add to the sweetcorn.
Halve the chilli. Scoop out the seeds and white bits for less heat (or leave them in). Finely chop the chilli. Drain the red onion. Add both to the sweetcorn with the runner beans.
Whisk 1 tbsp cider vinegar with 1 tbsp olive oil and a pinch of salt and pepper to make a dressing. Stir into the veg in the bowl. Tear in the basil leaves and toss to mix. Serve straight away.