Soften the onion, if using in a little water in a small pot. Set aside.In a separate, large pot, begin steaming the sweet potatoes. After about 5-10 minutes add the carrots; in 5 more minutes add the cauliflower. Steam for 5 minutes more. (They don't need to be cooked yet, just basically warmed.).Meanwhile, heat the oil over medium in a small frying pan;add the mustard seeds and saute till they begin to pop.
Add the remaining spices and cook on LOW heat for a minute or two, be very careful not to burn them.
Remove from heat.
Remove the steamer basket from the veggies, add the softened onion and stir in the spice mixture.
Add water.Bring to a gentle simmer and cook, stirring occasionally, for 20 minutes.When sweet potatoes are tender but not totally cooked, add the tomatoes and peas/peppers (a combo works fine). Simmer for another 15 minutes.
Serve over your favorite basmati rice, with the garnishes for a more Indian-style meal. Enjoy!