Use a medium bowl to prepare the marinade. Toss in minced or grated small onion, 1 tsp ground coriander seeds, 1 tsp paprika, 2 crushed garlic cloves, 1 tbsp salt, 1/2 tsp chili powder. Pour in 1 cup milk and 2 Tbsp olive oil. Mix all the ingredients well until the salt dissolves.
Start with pouring 1 tbsp of the marinade to empty bowl and stack chicken thighs by alternating with marinade. Continuously mix marinade so that its ingredients are evenly distributed between layers.
Cover the bowl with plastic wrap and leave it to rest for 12 hours in the fridge. Some recipes call for a shorter time to marinade but I believe this particular recipe for chicken thighs needs those extra hours to properly absorb spices and juice.
Preheat grill for 10-15 minutes. Grill 6-8 minutes each side on medium-high heat. It is really hard to indicate exact grilling time and temperature because there are many factors that influence grilling time. Everybody knows his or her grill’s hot and cool spots and grilling power. Depending on these factors grilling time wary. But please do not over-grill and turn the chicken into rubber.
It is ok to baste with leftover marinade only in the beginning stages of grilling so that it has enough time to cook with chicken. Basting towards the end of the cooking time is not wise.