Take 1 big tomato (or 2 mid-size tomatoes), blanch them and put them on the side. Add a spoonful of oil in a pot or cooker (depending on what you have). Chop 1/2 an onion fine and add it to the oil till they turn a little pink. When they do, add 1/2 teaspoon of ginger-garlic paste and mix this well. Now add the blanches tomatoes to your cooker/ pot. Add salt to taste, 1/2 teaspoon of turmeric powder, 1 teaspoon of chilli powder, and 1 cup of water. Cook this for 5-8 minutes if you're using a cooker or for 15 minutes if you're using a pot. It's essentially cooked at that point but it needs to be blended to get that consistency as you see in the picture. You can use a blender or a wire mesh to do this. Ideally the gravy is supposed to be sweet, so depending on how sweet your tomatoes are, you can add 1 spoonful of sugar while you blend it.
For the potatoes - boil water in a vessel and peel 3-4 big potatoes while this boils. Cut the potatoes into cubes of you preferred size. Boil these potatoes and add salt. Remove and drain after they are boiled. Take a vessel and add 1 tablespoon of oil, 1 teaspoon of mustard seeds(black balls as I call em), 1 small green chilli (chopped in 2 pieces or more depending on your preference). Be careful when you add this to the oil as it may all sputter, but it's supposed to sputter, so it's all good. Lower the flame and add the potatoes at this point. Sprinkle 1/2 a teaspoon of turmeric powder, 1 teaspoon of chilly powder and mix this up well. Cut the gas when you feel the potatoes are cooked. You don't want to over-cook this as it will get dryer as you cook it more, so it depends on the consistency you like.
For the potatoes there are a lot of variations ... like ... variation 1 - once the potatoes are boiled, add just the turmeric powder in the vessel with oil before adding the potatoes; variation 2 - once the potatoes are boiled, add the turmeric, chilli, cumin seeds and coriander powder to the oil before adding the potatoes; variation 3 - once the potatoes are boiled, add 1 table spoon of garam masala mix before adding the potatoes; etc.