Storecupboard tuna bean salad

Name: Storecupboard tuna bean salad
Prep time: 30 min
Total time: 30 min
Cuisine: British
Ingredients: 500g pack salad potato , such as Charlotte; 4 tbsp olive oil , plus extra for drizzling; 1 tbsp lemon juice; ½ tsp chilli powder; 1 plump garlic clove , finely chopped; 410g can cannellini bean , drained and rinsed; 1 small red onion or half a medium one, finely chopped; a good handful of parsley , chopped; 200g can tuna , drained; 110g bag mixed salad leaf and herbs