Heat 1 tablespoon vegetable oil in a large pot or Dutch oven to medium heat. Season the chicken with salt and pepper to taste, then cook 2-3 minutes per side to brown the pieces. Set aside onto a plate for now.
Heat 1 tablespoon vegetable oil in the same pot to medium heat, then add mustard seeds, coriander seeds, black peppercorns, cumin seeds, and fenugreek seeds. Cook 1-2 minutes to bloom the spices.
Transfer the bloomed spice mix to a food processor or blender. Add the onion, garlic, chili powder, ground turmeric, and salt. Blend to form a paste.
Add the curry paste to the same pot over medium heat. Add a bit more oil if needed. Cook 5-6 minutes to develop the flavor, stirring a bit.
Add the reserved chicken to the pot along with the red lentils, chicken stock, potato, carrots, and cinnamon stick. Bring to a boil, reduce heat, and cover. Simmer, stirring here and there, for 20-30 minutes, or until the lentils and vegetables are done to your liking. Remove cinnamon stick.
Serve over steamed rice if desired with red chili flakes or other garnish.