Heat mustard oil in the same cooking pot over medium-high heat until lightly smoking. Lower the heat, wait about 2 minutes, then add the crushed cumin and fennel seeds along with the cilantro stems and chopped green chilies. Fry gently, taking care not to burn the spices. Remove the pan briefly from the heat if needed.
Next, add the chopped onions and sauté until lightly browned, about 6–8 minutes. Do not let them turn dark brown. Add the garlic and ginger paste and sauté for another minute.
Then add the chopped tomatoes, coriander powder, turmeric, and cinnamon powder. Cook over medium-low heat for 3–4 minutes, until the tomatoes are soft and mushy but not dried out
Add the puréed spinach, salt, and about ? cup of the reserved spinach liquid. Mix well and cook on low heat for 8–10 minutes, stirring frequently. You’ll see bubbling as the spinach cooks and it's bright color starting to fade. Once properly sautéed, tiny oil bubbles will start appear on the surface of the sauce.
Add the paneer cubes, crushed kasuri methi (if using), ghee, sugar, garam masala, and heavy cream. Stir well to combine and cook for another 3–5 minutes, until the paneer is heated through and the curry shows gentle bubbling. You’ll also notice a thin layer of oil on the surface. Turn off the heat and let the palak paneer rest for 10 minutes before serving. Serve alongside hot flatbreads, steamed fluffy rice or jeera chawal.