Preheat oven to 200 C / 400F.
Place a large pot of water over high heat. Season with salt. Add potatoes and sweet potatoes. Boil, for about 5-6 minutes or until tender. Drain. Set aside.
Meanwhile, heat oil in a large frying pan or skillet over medium-high heat. Fry mustard seeds for few seconds.
Add onions and garlic. Sauté, stirring occasionally, for 2-3 minutes or until it turns light brown.
Add ground turmeric, ground cumin, garam masala and red chili powder. Sauté, stirring, for 30 seconds. Add spinach (if you are using). Sauté, stirring, for 1-2 minutes or until wilted.
Add potatoes and sweet potatoes. Toss. Stir fry for 1 minute. Remove from heat. Set aside to cool completely. Reserve 3-4 potatoes / sweet potatoes.
To make the egg mixture, crack eggs into a bowl. Whisk lightly. Add cream and coriander (cilantro) leaves. Whisk to combine. Season with salt and pepper.
Arrange potatoes and sweet potatoes in a 23-inch baking dish.
Pour egg mixture over potatoes and sweet potatoes in the baking dish.
Sprinkle with crumbled paneer or feta. Place the reserved vegetables on top.
Bake for 30-35 minutes or until frittata is set.