Pat the chicken thighs dry with paper towels and season them generously with salt and pepper. Add 2 tbsp neutral oil to a large saucepan or Dutch oven and set over medium heat. Once shimmering, add half of the chicken thighs and cook until golden - about 3-5 minutes - before flipping and cooking on the other side. Remove from the pan and set aside. Repeat with the remaining chicken and set aside.
Add the cumin seeds, black mustard seeds and fennel seeds to the pan. Cook until fragrant, only about 30 seconds to 1 minute. Add the onion along with a generous pinch of salt. Cook until softened and beginning to brown around the edges - 7-10 minutes.
Add the garlic, ginger, jalapeños and tomato paste. Cook until very fragrant and the tomato paste begins to stick to the bottom of the pan - about 3 minutes. Add the curry powder and cook for 30 more seconds.
Add the tomatoes and stir to combine. Cook until they begin to break down and release a lot of their moisture, about 3-5 minutes.
Cut the browned chicken thighs into bite-sized pieces and add them, along with any accumulated juices, to the pot. Pour over the chicken stock and bring to a simmer. Allow to simmer for 20 minutes, just to allow the chicken to completely cook and the flavours to meld together.
Stir in the cream. Taste to adjust for seasoning, adding salt and pepper where necessary. Serve alongside rice, roti or naan.
Notes
For less heat, remove some or all of the ribs and seeds from the chili.