Start by chopping the onion and chicken into strips. Prepare the curry paste ingredients by roughly chopping the garlic, ginger, chillies, and red pepper. Set everything out so you’re ready to cook with ease.
In a blender, combine the paste ingredients: onion, garlic, chillies, ginger, red pepper, spices, vinegar, tomato purée, salt, and chicken stock. Blend until smooth. This rich paste is what gives the curry its bold, deep flavor.
Warm the oil in a large pan and cook the chopped onion for 5 minutes until soft and golden. This brings out the onion’s natural sweetness and creates a base for the curry. Then add the curry paste.
Let the paste cook for a couple of minutes so the spices bloom and deepen in flavor. Now add your chicken strips to the pan. Stir well to coat every piece of chicken in the fragrant paste.
Allow the chicken to cook without too much stirring for about 10 minutes. This helps it develop some golden, caramelized edges. It also locks in flavor and gives you those gnarly, delicious bits.
Once the chicken is lightly browned, pour in the chopped tomatoes. Give everything a stir and bring to a simmer. The tomatoes will add tang and help mellow out the spice from the paste.
Let the curry simmer for another 15 minutes, uncovered, until the sauce thickens slightly and clings to the chicken. Stir occasionally, making sure nothing sticks to the bottom of the pan.
When the sauce has thickened to your liking, stir in the garam masala and lemon juice. This final touch lifts the flavor and gives the curry a warm, fragrant finish that’s hard to resist.
Serve hot over basmati rice. Optionally, stir in a handful of spinach or peas just before serving for a pop of color and added nutrition.