Peel the potatoes, and cut into long quarters or eighths. Place the potatoes in a small saucepan and cover with water. Add salt. Bring to a boil, then simmer with the lid tilted at a diagonal until the potatoes are tender. Drain.
Meanwhile, slice the onion. Roughly chop the garlic. Slice the ginger into a few planks. Heat the oil in a large skillet with tall sides. Cook the sliced onions until softened. Add the garlic and ginger and cook about 2 minutes. Combine the spices in a spice grinder (I used an old coffee grinder). Pour the ground spices over the onion mixture and turn off the heat. Let the spices warm up 1-2 minutes in the skillet.
Then pour the spiced onion mixture into a blender. Add the tomato sauce or tomato paste and about a half cup of water. Blend until pureed. This creates a masala paste. Pour the paste back into the skillet. Add another 1/4 cup of water to the blender and pulse to get the rest of the paste clinging to the sides of the blender. Add to the skillet.
Chop the chard. I stack the chard leaves on top of each other, roll them up tightly, then slice the roll thinly, which is called a chiffonade technique. Then I chop crosswise across the green ribbons a few times to get a rough chop. Add the chopped chard to the skillet.
By this time, the potatoes should be done. Drain the potatoes then add them to the skillet. Stir, turn on the heat to medium, and cover with a lid. When starting to bubble, lower the heat to very low and let simmer about 20 minutes.
At this point, I made a pot of rice with the rice cooker, and when the rice is finished, the Saag Aloo is finished simmering too. Turn off the heat, sprinkle over a teaspoon of garam masala, stir, then taste for salt. Adjust for balance.
Serve alongside other curries or a dal, or just with rice. Enjoy.