Add the cumin seed, cardamom, star anise, white mustard seed, cloves and coriander seed to a pan over very low heat. Toast until fragrant. Remove from the pan and grind into a fine powder. In the same pan, add the Bengali Panch Phoron and toast until fragrant. Do not grind.
Add 2 - 3 tablespoons of oil to a pan over medium heat. Add the Panch Phoron to flavor the oil. Once they begin to sputter and crackle, add in the onion with a heavy pinch of kosher salt. Cook until slightly softened.
Add the serrano chilis and cook until they lose the raw chili smell. Add the minced garlic and cook for 1 - 2 minutes, then repeat with the ginger.
Add the ground dried spices and the kashmiri chili powder, followed by the garam masala. Mix to combine.
Add the hot red chili powder, white pepper, turmeric, ground ginger and salt and stir to combine.
If needed during cooking, add a couple teaspoons of water to prevent sticking.
Add the canned tomatoes and mix well. Cook for 2- 3 minutes until the tomatoes start to break down. The add the Methi leaves if using, and cook for another 2 - 3 minutes.
Remove from the heat and cool for a few minutes.
Meanwhile, heat your water bath to 165°F. Place the chicken thighs in your vacuum bag and pour the curry mixture over. Make sure the sauce coats the chicken on all sides than vacuum seal. Add to the water bath and cook for 2 hours.
Once the sous vide cooking is complete, pour the chicken and sauce into a large bowl. Serve with basmati rice, Kachumber salad and raita.