1 Granny Smith apple, cored, cut into large pieces
Juice of 1 lime
1 (1") piece ginger, peeled
2 garlic cloves, peeled
1 cup cilantro leaves with tender stems
¼ cup mint leaves
1/3 tsp. (or more) sugar
1–2 green Indian, Thai, or serrano chiles
Filling and assembly:
1 Tbsp. vegetable oil or ghee
1 tsp. black mustard seeds
4 fresh curry leaves
1 tsp. cumin seeds
¼ tsp. ground coriander
¼ tsp. ground turmeric
A pinch of asafetida (optional)
1–2 green Indian, Thai, or serrano chiles, thinly sliced
1 large cooked russet potato, peeled
¼ tsp. kosher salt
½ cup chopped cilantro
1 tsp. chaat or sandwich masala (such as Spicewalla Chaat Masala or MDH Chunky Chat Masala)
8 (½"-thick) slices Pullman or other soft sandwich bread
1 small red onion, finely chopped
½ cup finely chopped green bell pepper or seeded plum tomatoes
8 oz. Monterey Jack or Emmenthal cheese, sliced, or 8 slices white American cheese
Room-temperature ghee or unsalted butter (for sandwiches)
Ketchup (for serving)
Purée apple, lime juice, ginger, garlic, cilantro, mint, sugar, a pinch of salt, and 1–2 chiles, depending on how spicy your chiles are and your heat preference, in a blender, scraping down sides and adding water by the tablespoonful, until a smooth paste forms. Taste and season with more salt or sugar if needed. (You should have about 1 cup, which is more than you’ll need for the sandwiches. Color will fade as it sits.)
Do ahead: Chutney can be made 3 days ahead. Transfer to an airtight container; cover and chill.
Filling and assembly:
Heat oil in a medium cast-iron or nonstick skillet over medium. Add mustard seeds and cook until popping, about 30 seconds. Add curry leaves and cumin seeds. Cook, stirring with a wooden spoon, until slightly darkened in color, about 1 minute. Stir in coriander, turmeric, asafetida (if using), and 1–2 chiles. Crumble potato into pan in large pieces and add salt. Cook, coarsely mashing and tossing potatoes to coat, until warmed through, about 2 minutes. Fold in cilantro and chaat masala. Transfer filling to a bowl. Wipe out pan and reserve.
Arrange 4 slices of bread on a cutting board. Spread some chutney over each slice. Divide filling among slices and top with red onion, green pepper, then cheese. Spread 1 side of remaining 4 bread slices with more chutney and close up sandwiches. Spread ghee over outside of top slices of bread.
Heat reserved pan over medium. Working in batches if needed, melt a small knob of ghee in pan, then arrange sandwiches, buttered side up, in pan. Cook until golden brown underneath, about 4 minutes. Turn over and cook, pressing down occasionally with the back of a spatula or the bottom of a saucepan, until cheese is melted and other side is golden brown, about 4 minutes.
Serve toasties with extra chutney and ketchup alongside.
Note: Sandwiches can be made in a panini or sandwich press as well, or ideally a toastie maker.