Mixed bean casserole

1 tbsp olive oil
1 small onion, finely diced
1 clove garlic, crushed
1 stick celery, finely sliced
1 medium carrot, diced
1 large red pepper, seeds removed and discarded, finely diced
½ tsp chilli powder
½ tsp ground cumin
½ tsp smoked paprika
½ x 400g can black beans, drained and rinsed
½ x 400g can chickpeas, drained and rinsed
1 x 200g can sweetcorn, drained and rinsed
½ x 400g can chopped tomat...