Put a pot of salted water on to boil and add the potatoes. Boil until soft, 20-25 minutes.

Place an oven rack in the second highest slot. Preheat the oven to 350F/180C.

While the potatoes boil, make the chutney:

Place the mint, cilantro, lemon juice, salt, garlic, and hot pepper in a food processor. Process, then take a taste, being mindful of the blade. Don’t expect this to taste very good raw; it will improve with cooking. Does it need more lemon or hot pepper? It shouldn’t need liquid, but if it does, add small amounts of sunflower or olive oil. Be very careful about adding salt; you’ll need very little. Scrape into a bowl and set aside.

No processor? You can make the chutney with a mortar and pestle, or a sharp knife and a cutting board.

Make the spicy mashed potatoes:

If you have a preferred mashed potato recipe, by all means use it. If not, I kept things simple here, mashing the potatoes, then adding the buttermilk. Feel free to add more buttermilk if you feel the mash needs it.

Add the chili powder, turmeric, ground cumin, salt, sumac or lemon juice, and black pepper to the potatoes. Stir in the grated cheese. Don’t worry if the cheese doesn’t melt. It will during the final oven blast.

Make the sandwiches:

You’ll need an baking tray for this step, ideally a heavy one. For ease of cleaning, you might want to use tinfoil or a silicon baking mat. Smear either the mat or the baking sheet with butter. Now butter two slices of bread and place them butter-side-down on the baking sheet.

Put down a layer of mashed potato on the bread, then the chutney, tomato, and onion, if using, It’s important that the chutney be added now. Otherwise, sogginess.

Butter two pieces of bread on one side only. Close the sandwich butter side up.

Slide the sandwiches into the oven and allow them to cook 10-15 minutes. You’re looking for them to heat through completely. Once they do, turn your setting to broil and watch the sandwiches closely lest they burn. You want a delicate char atop the sandwiches. As this happens quickly, you need to stand watch.

Tamasin Day-Lewis suggests eating these sandwiches with catsup and potato chips, known in the U.K. as crisps. We wolfed them down without accompanyment, though something green to offset all that richness probably wouldn’t go amiss. Napkins of the wet washcloth variety are a good idea here.

The Bombay/East Bay Vegetable sandwich reheats beautifully in a low oven. Don’t microwave it unless you enjoy rubbery food. Don’t even think of freezing it.

Notes: Spicing of this dish can range from mild to wild. I took the middling route here.

Sumac is present because the potatoes benefit from an acid component. Lemon juice is fine if sumac eludes you. Tamarind would work well, too.

Go easy on salting the chutney. I used fine sea salt the first time I made it, and the result was inedible. Maldon salt is deliberately specified here. If you can’t or won’t spend $11 on salt, understood: add fine salt sparingly–less than 1/8th of a teaspoon–and taste as you go. The same goes for adding hot pepper: start with small amounts. You can always add to a dish, but you can’t take away.

The original recipe calls for Gruyère cheese. I used sharp cheddar to excellent effect. If you don’t want to grate your cheese, use quality sliced cheddar. Please avoid anything claiming to be “cheese food.”

Day-Lewis specifies thinly sliced onion in her recipe. I prefer it without, but mention it here as an option in case you do.