Preheat the oven to 400°F.
Separate the green top of the leek from the bottom. Cut the bottom into thick circles, about 1 cm thick per piece. Fold and thinly slice the green top, turning it into slivers. Set both in a large bowl.
Thinly slice the brussels sprouts using a knife or mandolin. The thinner the better. Set into the same bowl.
Cover the leeks and sprouts with water and give a few good mixes with your hands to get any dirt off. Drain the water and repeat as needed. Dry in a salad spinner.
Line one sheet pan with foil and the other with parchment paper. Spread the leek greens and brussels sprouts on the parchment paper side. Spread the leek circles on the foil. Drizzle both with olive oil and season with salt. Fold the aluminum foil over the top of the leek circles to encase them, and then stack the baking sheet with the greens on top. This is to get the leeks on the bottom pan rich and jammy, and the greens on top nice and crispy. Roast for 20-25 minutes at 400°F.
Thinly slice the shallots & grate the garlic. Add a generous glug of olive oil to a sauté pan and put it on medium heat. Add the shallots and cook for a good 15-16 minutes, or until they start to char and darken. Add in the garlic and red pepper flakes and cook another 1-2 minutes.
While the shallots are cooking, bring 2 & 1/2 cups of water to a boil. Season with salt and add in the buckwheat. Cook for 12-15 minutes or until most, if not all of the water is absorbed.
Whisk together the honey, mustard, tahini, 1/4 cup water and juice from one lemon. Pour this into the shallots, cook for 1-2 minutes before turning off the heat. Stir until the sauce is thick and glossy. Season as needed.
Add in the buckwheat to the shallot sauce. It should still have a bit of water in the pot, so feel free to add that in too to help the sauce cling to the grain.
When the vegetables are done roasting, add them into the buckwheat as well. Mix until everything is thick and creamy.
Finish with a squeeze of juice from the other lemon, and add a few cracks of black pepper. Serve warm!