Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat (if you plan to double the recipe, definitely use a pot instead of a skillet so you have room for all the spinach!). Add the onion, garlic, and ginger and saute until softened, about 4 to 5 minutes, stirring regularly, and lowering heat to medium-low if it starts to brown too quickly.
Over medium heat, add the salt, coriander, garam masala, turmeric, cumin, cardamom, and chili powder, and stir on medium-low heat until fragrant, about 30 seconds or so. Dissolve the tomato paste in the water by stirring vigorously together in a measuring cup or bowl. Then stir into the spices in the pan, creating a thick spiced sauce, about 1 minute.
Add the spinach and gently stir to combine, as the skillet will be very full. Cover and raise the heat to medium-high, allowing the spinach to start to wilt. Occasionally remove the lid, stir, and then cover again until most the spinach has begun to wilt, about 3 to 5 minutes (can depend on the size of your spinach leaves). Remove the lid and finish stirring the spinach until it is completely wilted but still bright green, another minute.
Transfer spinach mixture to a blender and puree until relatively smooth but still a bit chunky (you don’t want it to be super smooth, a bit of texture is good). Taste and adjust salt and spice if necessary.
Meanwhile, wipe the skillet clean with a paper towel, return it to medium-high heat, and add 1 tablespoon of oil. When the oil is hot, add the cubed chicken pieces, sprinkle salt over the top to season, and cook until almost completely cooked through, about 5 to 7 minutes. Pour the spinach mixture back over the chicken, using a rubber spatula to get every last bit out of the blender, and cook for another 2 to 3 minutes to allow the flavors to meld and the chicken to finish cooking. Stir in the yogurt, and serve immediately with basmati rice and/or naan.
