In a deep skillet, heat oil over medium heat (or omit and use a non-stick skillet). Add cumin and mustard seeds and cook for 1 to 2 minutes until mustard seeds begin to sizzle and pop. Add pepper, garlic, ginger, and onion. Stir and let cook 2-3 minutes until softened and fragrant.
Add the potato and mushrooms, if using. Stir to coat. Add coconut milk, turmeric, and black pepper and stir well. Cover pan and let cook for 10-15 minutes until potatoes can be pierced with a fork easily. If a more wet curry is desired, you can stir in ½ cup of water or broth. Add peas, cilantro, salt and sweetener (if needed), lime or lemon juice, and stir. Cook 1 to 2 minutes more and serve immediately.