Cut your chicken breast into small, bite-sized pieces around half an inch to one inch wide. Drizzle the cubes with olive oil, then season generously with salt and pepper. Toss everything together until each piece is evenly coated.
Heat your skillet over medium heat and cook the chicken in two or three batches so the pan isn't overcrowded. Cooking in batches keeps the chicken from steaming, which is what actually gives it that golden, browned exterior. Once done, set the chicken aside on a plate.
Add your diced onion straight into the same skillet, adding a splash of water or a little extra oil to loosen up any browned bits. Sauté for two to three minutes until the onion turns soft and translucent. This step builds a ton of flavor into your base.
Stir in the diced tomatoes with their juices, tomato paste, pumpkin puree, coconut milk, and lemon juice. Mix everything together until it forms a smooth, thick sauce. At this point your kitchen should already start smelling incredible.
Sprinkle in the curry powder, chili powder, cumin, mustard seed, coriander, cinnamon, and brown sugar. Stir well so every spice gets fully incorporated into the sauce rather than clumping in one spot.
Return your browned chicken back into the skillet, stirring it into the curry sauce. Cover with a lid, reduce the heat to low, and let everything simmer gently for 10 to 15 minutes until the chicken is fully cooked through.
Spoon your finished curry over a bowl of warm rice or your grain of choice. Add any toppings you like, then dig in while it's hot and fragrant.