Wash and clean the raan thoroughly and pat it dry. WIth a sharp knife, make deep scores at various places in the raan for the marinade to go through.
Dry roast the whole spices in a frying pan till they release some aroma.
Then in a grinder, make a paste of the spices along with all the ingredients listed under marinade. Also add 2 tbsp of mustard oil to this paste and grind. The paste should be a smooth fine paste. In case you feel that the sourness is too much, add a bit more of sugar to balance it.
Now place the raan in a roasting tray.
Smear the marinade all over the raan. Not an inch of space should be left. Use all the marinade properly over the raan and massage the meat.
Now leave the raan overnight in the refrigerator. If not overnight, then at least for 4 to 5 hours.
Before placing in the oven, take the raan out of the refrigerator and bring it to room temperature.
Pre heat the oven to 180 degrees.
Drizzle some mustard oil over the raan and sprinkle some sea salt and then place it in the oven.
While the raan is on the oven, take the potatoes and wash them well. Poke the potatoes with a fork all over. Then in a pout of boiling water, put some salt and then boil the potatoes for 8 minutes.
After 8 minutes, drain the water out and keep the potatoes aside. Cut the onions into quarters.
After about half an hour of the raan in the oven, take the tray out and baste the raan with the juices which are flowing out along. Baste the raan with some more mustard oil and place the raan back into the oven.
Keep basting every half hour for best result. After 2 hours of oven time, increase the temperature to 200 degrees and place the onions and the potatoes all around the raan.
Sprinkle some sea salt over the potatoes and the onions. Drizzle mustard oil. Then put the tray back into the over for roasting for another hour.
In case you feel that the meat has become tender in less than an hour, then do as per that. The potatoes and onions take about 25- 30 minutes to cook.
Take the tray out of the oven when the meat becomes tender. Check with a fork if it is going in smoothly.
Take the raan and the vegetables out of the tray and keep them aside.
With a spoon, scrape out masalas and the jus from the tray and put them in a sauce pan.
Don't scrape out the really burnt bits.
Then add a half a cup of water and half a cup of red wine and let it simmer for 8 - 10 minutes over medium to low heat.
Make a slurry with the flour and 2 tbsp of water and put it into the sauce and stir properly so that no lumps are formed.
Give it one boil and season as per your taste. If the consistency is too thick, then add some more water.
Serve the raan with the vegetables and the sauce on the side.