Heat 3 tablespoons oil in a large, deep-sided skillet over medium heat. Add ½ teaspoon cumin seeds; cook, stirring often, until fragrant, about 1 minute. Stir in chopped onion and serrano; cook, stirring occasionally, until the onion is translucent, 2 to 3 minutes. Add 2 teaspoons garlic and 2 teaspoons ginger; cook, stirring often, until fragrant, about 1 minute.
Add 1 teaspoon coriander, 1 teaspoon salt, ¾ teaspoon chili powder, ½ teaspoon garam masala and ¼ teaspoon turmeric; cook, stirring constantly, until fragrant, about 2 minutes. Stir in 1 can tomatoes; bring to a simmer over medium heat. Cook, stirring occasionally, until slightly thickened, about 2 minutes.
Add diced potatoes; cook, stirring often, until well coated, about 2 minutes. Stir in 2¼ cups broth; increase heat to high and bring to a boil. Cover and reduce heat to medium. Simmer, stirring occasionally, until the potatoes are fork-tender, 15 to 18 minutes.
Stir in 4 cups spinach, 2 cups kale and 1¼ cups peas; cook, uncovered and stirring often, until the greens are wilted and the peas are bright green, about 2 minutes. Garnish with cilantro leaves, if desired.
