Method
First, prepare the tahini dressing. You can make this well ahead of time. Combine ingredients in a bowl and whisk. Add more tahini or water to achieve the right consistency—spreadable but not pourable. Add salt or vinegar to taste. Set aside until you are ready to serve.
Rinse the buckwheat using a fine mesh strainer until water runs clear. In a medium-sized pot, bring 1½ cup of water to boil over high heat. Stir in the buckwheat until the water boils again, then reduce to a low simmer. Cook until tender or water is absorbed, approximately 15 minutes. Remove from heat, drain and set aside to cool.
Meanwhile, in a large skillet over medium-high heat, cook the shallots in olive oil. Stir frequently to avoid burning the shallots, around 5 minutes. Next, add the mushrooms and cook, tossing until golden brown, about 3 minutes. Season mushrooms with salt and pepper, and cook for a further 3 or so minutes.
Reduce heat to medium, add the thyme and garlic. Remove from heat, squeeze over the lemon juice and fold through the herbs. Transfer the mushrooms to the buckwheat, and fold through to combine.
To assemble, plate the tahini mustard dressing first, then layer on top the buckwheat salad. Garnish with extra herbs and extra toasted seeds. Serve immediately.