Finely chop half the cucumber. Thinly slice the remaining cucumber. Put the sliced cucumber and 2 tsp white vinegar in a bowl, season with salt and pepper and toss to combine. Set aside to pickle, tossing occasionally, until needed.
Step 2 — Start onion rice
Thinly slice the onion. Crush or finely chop the garlic. Heat 1 tbs oil in a medium deep frypan over medium heat. Cook the onion, stirring, for 5 mins or until soft. Add the garlic, rice and half the masala seasoning and cook, stirring, for 30 secs or until the grains are well coated.
Step 3 — Finish onion rice
Stir 375ml (1½ cups) water into the rice mixture and bring to the boil. Reduce the heat to medium-low and cook, covered, for 12 mins or until tender and the water is absorbed. Turn off the heat and stand, covered, for at least 5 mins.
Step 4 — Marinate chicken
Meanwhile, cut slits 2cm apart across each chicken thigh, taking care not to cut all the way through (see cooking tip). Combine the remaining masala seasoning and 1 tbs yoghurt in a large bowl and season with salt and pepper. Add the chicken and turn to coat.
Step 5 — Cook chicken
Heat 1 tbs oil in a medium frypan over medium-high heat. Cook the chicken for 5-6 mins each side until cooked through.
Step 6 — Make yoghurt and serve
Combine the chopped cucumber and remaining yoghurt in a bowl and season with salt and pepper. Stir the spinach into the rice. Divide the onion rice and tandoori chicken among plates. Serve with the drained pickled cucumber and cucumber yoghurt.