Heat oil in a large skillet over medium heat. Add cumin or mustard seeds. Cook until seeds turn golden brown (for cumin) or gray (for mustard) and release their aroma, about 1 minute. Seeds will "pop" as they cook. Watch seeds closely to be sure they don't overcook. Add potatoes into the skillet. Reduce heat to medium-low, cover and continue cooking for 15 minutes. Stir occasionally to prevent the potato mixture from sticking to skillet. Add cabbage and continue cooking until vegetables are semi-tender. Add spices and stir to evenly coat vegetables. Cook until vegetables are tender, about 10 minutes. Stir in cilantro, lemon juice and tomatoes and serve immediately. Makes 2 to 3 servings