Heat the grapeseed oil in a large, heavy skillet or soup pot. Brown the chicken on medium-high heat on both sides, about 5-7 minutes per side, until it has a nice golden brown color. Remove the chicken and set aside.
Add the leeks, onion, garlic, red bell pepper, red pepper flakes, and oregano to the pot. Sauté on medium heat until the peppers and onions have softened, 10-12 minutes. Add the white wine and cook for 3-5 minutes. While the wine is cooking make sure to scrape the bottom of the pan so that you get all the nice crusty bits of chicken mixed back into the sauce.
Insert a sharp knife into the open can of plum tomatoes and cut up the tomatoes in the can (seriously – just swoosh the knife around in the can, it works, I promise). Add the plum tomatoes, including their juices, as well as the tomato sauce, the salt, the sugar and the Kalamata olives to the pot. Add back the browned chicken. Simmer over medium-low heat for 60 minutes.
Just before serving add the lemon juice. Serve over rice, flat noodles or mashed potatoes.
