Place the potatoes into a large pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until tender, about 20 minutes.
Drain and allow to steam dry for a minute or two.
Meanwhile, combine cumin seeds, ground coriander, ground turmeric, curry leaves, and sliced green chile in a bowl. Set aside.
Place vegetable oil and mustard seeds in a large skillet. Cover and cook over medium heat until the mustard seeds begin popping, then reduce heat to low. When the popping stops, quickly add the cumin mixture and replace the lid; cook until the herbs make a hissing sound.
Mix in the green bell pepper, garlic, and onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Stir in the spinach, corn, and peas until heated through. Stir in the baked beans. Toss in the cilantro, and season with chili powder, 1/2 teaspoon of salt, and pepper. Divide vegetable and bean mixture evenly into two 9-inch round pie pans.
Combine potatoes, butter, milk, and 1 pinch of salt in a bowl. Mash until smooth.
Spread mashed potatoes over the bean mixture using a fork to create small peaks that will become crisp during baking.
Bake in the preheated oven until potatoes are golden brown, 20 to 25 minutes. Cool for 5 minutes before serving.
