Preheat the oven to 200 degrees Celcius. Place the cut potatoes in a large pot along with 1 teaspoon of the salt, turmeric, and enough water to cover. Bring to a boil, then reduce heat and simmer for 5 minutes. Strain the potatoes from the water and transfer to a large bowl.
Mix the oil, mustard seed, curry powder, garam masala, and remaining salt in a small bowl and then add to the potatoes and mix properly to make sure all of the potatoes are covered.
Transfer the potatoes to a foil-lined rimmed baking sheet and roast in the pre-heated oven for 20 to 40 minutes, flipping the potatoes once half-way through.
During the last 15 minutes, sprinkle over the sliced red onion and quartered tomatoes and put back into the oven to continue cooking
Remove from oven once the potatoes are fork-tender. Serve hot and sprinkle with the chopped fresh coriander.