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Heat ¼ cup oil in a small saucepan over low heat until just warm. Add mustard seeds and cook until they start to pop and sputter, which should be a matter of seconds. Quickly remove pan from heat, add curry leaves, and toss to coat—they should crisp immediately from the residual heat. Set spiced oil aside.
Mix together onion, green chiles, yogurt, cilantro, black pepper, and chili powder in a small bowl; season with salt. Spread yogurt mixture over 6 slices of bread and top with remaining slices to make 6 sandwiches.
Heat 1 tsp. olive oil in a large skillet over medium heat until shimmering. Reduce heat to low and cook 2 sandwiches until undersides are crisp and golden brown, about 3 minutes. Turn, add another 1 tsp. olive oil to skillet, and cook until the second side is crisp and golden brown, about 3 minutes. Transfer sandwiches to a platter or plates; repeat with remaining sandwiches and 4 tsp. oil.
Drizzle reserved spiced oil evenly over sandwiches. Cut in half and serve with chutney and/or ketchup alongside if desired.
Do Ahead: Yogurt filling can be made 1 day ahead. Cover and chill.