Cheese and corn polenta loaf

Name: Cheese and corn polenta loaf
Prep time: 25 min
Cook time: 1 h 5 min
Total time: 1 h 30 min
Cuisine: American
Ingredients: 2 Tbsp unsalted butter, plus extra 180g, melted; 350g corn kernels, fresh or frozen; 2 cups plain flour, sifted; 200g instant polenta; 1 Tbsp and 1 tsp baking powder; 3 tsp ground cumin; 2 Tbsp caster sugar; 250g grated cheddar cheese; 1 1/2 tsp sea-salt flakes; Freshly ground black pepper, to season; 2/3 cup milk; 200g thick Greek yoghurt; 5 free-range eggs, separated; 2 tsp honey; 1 white onion, sliced into 1cm rings; 1 red onion, sliced into 1cm rings; Softened butter, to serve