First clean, wash and chop all the vegetables as mentioned in the list above. Keep it aside.
To begin, heat oil in a heavy bottom pan over medium heat, add chopped onion and garlic. Saute for a couple of minutes until onions are softened. No need to brown the onions and garlic here.
To this add chopped tomatoes and cook till they turn soft and mushy. Once done add roughly torn basil leaves. Transfer this to a bowl and keep it aside.
Next, in the same pan add some olive oil. Add chopped carrots, French beans, Zucchini, Broccoli, Mushrooms and Bell peppers one by one. Saute for 3 - 4 minutes until vegetables are lightly softened.
After a while, you will notice liquids begin to release from veges and it will look saucy.
Season the vegetable mixture with salt and pepper as per your taste. Stir the vegetable mixture, add water if required and allow it to simmer for about 5 to 7 minutes until vegetables are cooked upto 90 %.
To this add prepared tomato sauce. Keep stirring until the vegetables are incorporated well with the tomatoes.
Meanwhile let's prepare the mashed potato topping for Shepherd's pie.