Peel potatoes and carrots ( if desired ), peel sweet potato, wash and cut potatoes in half, cut carrots and sweet potato into large sections and place all into a large saucepan, add the one and a half cups of water and place on stovetop and bring to a boil, turn heat down to simmer once boiling and cook for about 15 minutes until tender ( remember to check the saucepan or it may burn if water evaporates to quickly ). Peel onion and garlic, wash, dice garlic finely and onion ( your choice, either sections, large or small dice ). Heat the oil on medium heat in a fry pan, when hot add onion and saute for 3 minutes then add the garlic and saute for 3 minutes more. Add the frozen corn, then the peas to the saucepan and cook for 5 minutes, all water should of evaporated by now so add the butter or margarine and then the sauted onion and garlic and toss through to coat all vegetables.