To make the potato pea filling, heat oil in a pan over medium heat. Add cumin seeds and let them sizzle for 30 seconds. Add ginger and green chilis and stir for a couple of minutes. Then add spices and salt. Stir for 30 seconds before adding peas.
Cook peas for another few minutes before adding the potatoes. Once you add the potatoes, mix them with everything else for about 5-7 minutes. Set aside so they can cool before you fill the momos.
To fill the momos, set aside a small bowl with water. Place half a tablespoon or so of filling in the center of the wrapper or as much filling that fits. Gently spread some water with your finger on the edges of half the wrapper. Fold the wrapper almost in half, making small pinches on one side of the wrapper until the entire side is folded and ready to seal. This is just one way to do it! Feel free to do whatever you like.
To prepare the momos, you can steam or pan fry them. Steam in a greased steamer basket with plenty of simmering water. Cover and steam for 3-4 minutes or until they are soft. To pan fry, simply fry them for a few minutes per side in 2 tbsp of neutral oil. They should be golden and crispy on each side. For the chili momo sauce, heat oil in a pan or wok over medium heat. Add garlic, green chilis, ginger, and scallion whites. Stir for a few minutes before adding onion and bell pepper. Stir for a few more minutes before adding ¼ cup water, covering, and letting it cook for 5 minutes or until the peppers are cooked.
Uncover and add tomato paste, ground coriander, black pepper, and red chili powder. Cook for 2-3 minutes before adding the sauces: chili garlic sauce and black bean garlic sauce, if using. Finish with soy sauce and a cornstarch slurry.
Toss the momos in the sauce, garnish with scallion greens, and serve immediately.