the base is pretty much gora ghee (butter + olive oil) with either very, very finely cut or blended onion and garlic. you can add chilis to this if you want more spice, but cut out the seed if you want chili flavour without the heat.
anyway, you fry the masala paste for about five minutes, ideally while covered, on a low-moderate heat. i have a knackered cooker so go for a low heat if yours is decent.
whatever spices you use, i would suggest using a good bit (at least a half tsp) each of turmeric, coriander and cumin (maybe less turmeric since it gets overpowering), along with obligatory black pepper and salt - adjust these to your taste. you always need more salt since blended baby spinach is a lot of moisture. anything else is to your taste. i like to add plenty of cinnamon for autistic wellness raisins. add the spices to the masala paste and fry only until aromatic, it takes less than a minute. burnt spice is shit.
the original recipe i work to (yt, its a fat pakistani guy) says don't let the paste catch but tbh i find that a little touch of burn makes onion taste better. it all depends on you.
i slice the tenderloin thin as fuck, as though you were making tiny schnitzls. obviously you don't need to pound it flat like with schnitzl but you get what i mean. the point is that the paste is still cooking so you want the meat to seal as fast as you can get it to. keep everything moving, the meat needs to be coated.
blend or crush some cherry tomatoes (you can use other tomatoes of course, its just out of a supermarket they're the most flavourful) with about 30-50g of fresh coriander (if you want to make it like i do. its not really necessary) and mix with about a tbsp of tomato puree to make your passata. if you are blending spinach then use less tomato because too much moisture will make it a fucker to reduce - at least if you have a shit cooker like mine.
add the passata and wait until it is steadily simmering, then pour in the blended spinach. only use half a bag because the rest will be added later. you'll probably need more salt at this point. you'll know if its too watery.
cook it all on a low heat while half covered for as long as you think you need to cook it, until it thickens. then add the remaining spinach leaves and cook for another five minutes. at this point you can stir in a couple tbsp of plain yoghurt but it really isn't necessary. depends on what kind of curry you like, i suppose.
i hope this was halfway coherent, i sort of battered it out in a stream of consciousness and im not going to revise or do it again. sorry