Heat the oil in a wide pan or braiser over medium heat. Add the onion and salt, and cook while stirring occasionally until the onions have softened and are starting to brown, about 15 minutes—adjust the heat toward medium-low if they begin to brown too quickly.

Reduce the heat to medium-low. Add the garlic and ginger, stirring constantly until they are lightly browned and fragrant.

Add the garam masala, coriander, cumin, red chili, and turmeric, stirring for a few seconds to toast the spices. Add the tomato paste and water and stir until well combined. Increase the heat back to medium and cook until the mixture has thickened significantly, about 10 minutes.

Transfer the onion and tomato mixture to a blender after letting it cool slightly, then process until smooth. Use immediately in a gravy dish or store for later use.