Mexican bean casserole

Ingredients

• 2 teaspoons vegetable oil
• 1 medium onion, diced
• 160g frozen pepper, thawed, or ½ fresh green pepper and ½ fresh red pepper, sliced
• 1 clove garlic, crushed
• ½ teaspoon chilli powder (optional)
• ½ teaspoon ground cumin
• Freshly ground black pepper, to taste
• 1 x 400g can mixed beans or pulses in water, drained
• 1 x 400g can chopped tomatoes
• 1 x 198g can salt-free sweetcorn, drained, or 160g frozen sweetcorn, thawed
• 200ml water