Place rack in middle position and heat to 375 degrees. Combine the garlic, rosemary, coriander, salt, pepper, and olive oil in a bowl and mash together to make a paste.
Smear the paste all over the roast and tie into a uniform shape so the roast cooks evenly.
Lightly dust the roast with the flour and put it on a wire roasting rack. Stick the roast in the over and cook until an instant read thermometer reads 135 degrees (medium rare) about 50 minutes.
While the roast is in the oven, put all the ingredients for the yogurt sauce in a bowl and mix and put it in the fridge.
Take the roast out of the oven and let it rest for about 15 minutes. This allows the juices to redistribute back into the meat and is a key part of having a nice tender juicy roast.
Slice and serve with the sauce.