Bateta Vatana Nu Rasawalu Shaak Gujarati Potato Pea Curry

Name: Bateta Vatana Nu Rasawalu Shaak (Gujarati Potato & Pea Curry)
Ingredients: 2 medium white potatoes (400 grams); 2 teaspoons vegetable oil; 1/8 teaspoon black mustard seeds; 1/8 teaspoon cumin seeds; 1 Thai chile pepper (10 grams), sliced into thin round slices (optional); 1 Roma tomato (100 grams), diced into 1/2-inch pieces; 1/2 teaspoon grated ginger; 1/8 teaspoon ground turmeric; 1 teaspoon ground coriander; 1/4 teaspoon ground cumin; 1/4 teaspoon red chile powder; Salt, to taste; 1/4 cup frozen peas; 1/2 teaspoon garam masala; Cilantro, for garnish