Heat oil in a 10-inch, non-reactive saute pan over medium-high heat. Add cumin seeds, fennel seeds, and mustard seeds, cover pan with a splatter screen, and cook, stirring occasionally, until they begin to pop. Once they begin to pop, turn the heat down to medium, and add turmeric, onion powder, coriander, cinnamon, garlic, and chiles. Saute until garlic turns golden brown in color, 3 to 5 minutes. Gently add the vegetables, sugar, salt and pepper, if desired, and cook until vegetables are heated through, 3 to 5 minutes. Remove vegetables from heat, pour into bowl with yogurt mixture, and stir to combine. Remove chiles if desired, and serve immediately.