Begin by adding the onion, celery, garlic, sweet potato, carrot and red pepper to a large pan, with a dash of oil and cook on a low setting for 10 minutes to soften. Ensure a lid is secured tightly to retain all the heat and steam.
Once the contents of the pan have cooked for 10 minutes, add the lentils, stock, Worcester sauce, parsley, thyme and seasoning, give everything a good stir to combine and bring to the boil. Leave to simmer for up to 40 mins as per packet instructions.
While the lentils are cooking, prepare the topping.
Place the potatoes in a pan, top with water and bring to the boil. Turn down the heat to simmering point and leave the potatoes to cook for about 20 minutes, until soft.
Once the potatoes are cooked, drain the water, add the butterbeans and the milk a splash of olive oil and salt then mash until smooth and creamy.
Once both the filling and topping have cooked/been prepared, add the spinach to the filling pan and stir through then add the filling to a suitable sized casserole dish, then add the topping using a spoon to smooth over. Place the casserole dish in the oven and cook for 20-30 minutes until the potato starts to crust on top.
Serve immediately with extra vegetables of your choice.