Toss the shrimp with 1/2 teaspoon garam masala in a medium bowl; season with salt and pepper. Heat 1 tablespoon olive oil in a large pot or Dutch oven over high heat. Add the shrimp in a single layer and cook until the edges are golden, about 30 seconds per side. Remove to a plate with a slotted spoon; set aside.
Add the remaining 1 tablespoon olive oil to the pot, then add the onion and garlic. Cook, stirring occasionally, until the onion is lightly browned, 3 minutes. Stir in the tomato paste, the remaining 1 1/2 teaspoons garam masala and the cilantro. Cook, stirring to coat, 30 seconds. Stir in 3 1/4 cups water and the potatoes. Cover, bring to a boil and cook 5 minutes, then uncover and add the cauliflower. Cook over medium-high heat, stirring occasionally, until the cauliflower is tender, about 10 minutes.
Stir in the shrimp and peas; simmer 3 minutes. Season with salt and pepper. Top each serving with more cilantro. Serve with the naan.