Position rack in lower third of oven and preheat to 375°
1) Cook the freekeh according to the package instructions, about 45–50 minutes, using 1-1/2 cups of stock in lieu of the water; check the freekeh halfway through cooking, add additional stock if necessary. Set aside.
2) Spray a 4-quart casserole dish with nonstick vegetable oil spray and set aside.
3) Heat a cast-iron skillet or other heavy-bottomed skillet over high heat, add the coriander, cumin, and nigella seeds and toast, stirring, for 30 to 45 seconds, or until very fragrant. Transfer to a spice grinder or immaculately clean coffee grinder and grind the toasted seeds finely. Set aside. Return the pan to the heat, add the pine nuts and toast, stirring until they become light golden, about 1 minute. Transfer to a small container and set aside.
4) Place the same pan back over high heat and add the olive oil, heating until it shimmers. Add the onion and cook, stirring, for 3 to 5 minutes, until translucent.
5) Add the mushrooms, salt and cook, stirring, for 1 minute longer. Add the celery, carrots, and parsnips, and continue to cook until soft, about 3–4 minutes. Add the garlic and cook until just fragrant, about 1 minute. Deglaze with the wine and cook until slightly reduced, about 2 minutes. Add the raisins, thyme, ground spices, the sumac, pine nuts, toasted sesame seeds and stir. Add the cooked freekeh and stir until well combined. Add ¼ cup vegetable stock if mixture seems too dry. Set aside to cool slightly.
6) Add the tomato puree to the bottom of the casserole dish. Stuff the peppers with the freekeh mixture, gently pressing the mixture into the pepper until filled to the top. Flatten out the mixture with the back of a spoon. Do not overfill. Place the stuffed peppers into the prepared casserole dish, leaning them against each other as necessary so they stand up. Place the tops back on the stuffed peppers.You can use foil balled up to fill any areas to help them stand. Cover the casserole with foil and cook for 30 minutes. Uncover the casserole dish, cooking and add the remaining vegetable stock bake for another 30 to 40 hour, or until the peppers’ walls are fully cooked and very soft.