In a large frying pan (non-stick is good) over medium-high heat add 1 tablespoon coconut oil and the potatoes. You are browning the potatoes, not necessarily cooking them through. Do not move them for 3 to 5 minutes to let them brown. Turn over and brown on the other side for another 3 to 5 minutes. Remove the potatoes from the pan and set aside.
Add another tablespoon of oil to the pan. Over medium-high heat the oil and test with a couple of mustard seeds. If they start to sputter right away, the oil is hot enough. Add the rest of the mustard seeds and sputter for 30 seconds. Reduce the heat to medium and add the onions and 1/4 teaspoon salt. Cook until the onions are a golden-brown; about 5 to 8 minutes. Stir in the curry leaves and garlic and sauté for 2 to 3 minutes.
Add the chili and turmeric powders. Start with a lesser amount of chili powder. You can always add more to your taste. Stir for several minutes to cook the spices.
If your frying pan has a lid add 1/2 cup water, the potatoes, 1/2 teaspoon of salt or to your taste, and lemon juice. If you need to switch to a lidded dutch oven or sauté pan, do that now. Cover and cook the potatoes and masala mixture over medium-low heat for 10 minutes, or until the potatoes are soft but not mushy. Add water as needed. Taste for salt, heat, and acid.
Flavoring ideas for aloo masala Kerala style: add some minced green chilies to your taste. Play with the ginger are garlic. Either use 1 tablespoon garlic, 2 teaspoons of ginger OR 1 1/2 tablespoons of ginger garlic paste.