Preheat the oven to 200°C. Peel and chop the potatoes into rough chunks, then place into a pan of cold salted water. Bring to the boil, then simmer for around 15-20 minutes. Drain, mash until smooth, season with salt and pepper to taste, then set aside.
Place the broccoli, peas, carrots and baby corns into a medium pan of cold salted water over medium heat. Add one tablespoon of vegetable stock, bring to the boil and simmer for around 10 minutes. Add the lentils and tinned tomatoes and simmer for around 20-25 minutes, or until the sauce has thickened, stirring occasionally.
Peel and finely slice the onion and 3 garlic cloves then deseed and finely slice the chilli. Heat 1 tablespoon of olive oil in a large frying pan. Add the garam masala, sage, thyme, onions, garlic and chilli into the pan and fry for 5 minutes. Season to taste, then mix it into the sauce.
Transfer the sauce to a baking dish. Spread the mash over the top - you can get creative with the patterns for the top - use a fork to fluff it up or use a piping bag for more intricate designs. The rougher the surface, the crispier it will turn out. Place in the oven for 35-45 minutes or until the top is crispy/golden and bubbling.