Heat ½ cup water in a large pot.
Add onion and cook over medium-high heat for about 3 minutes, stirring occasionally, until onion begins to soften.
Reduce heat to medium. Add cauliflower and potatoes. Continue cooking, stirring often, for about 5 minutes.
Add water, ¼ cup at a time, if needed, to prevent sticking.
Toast cumin seeds, turmeric, coriander, ginger, cinnamon, and cayenne in a dry skillet over medium heat for about 2 minutes, stirring constantly.
Add spices to vegetables along with tomatoes and salt. Stir to mix, then cover and simmer for about 20 minutes until flavors are blended.
Stored in a covered container in the refrigerator, leftover Potato and Cauliflower Curry will keep for up to 3 days.
