Heat 1 tablespoon of butter in a dutch oven over medium-low heat.
Once melted, add the mushrooms and a pinch of kosher salt. Toss to coat the mushrooms in the butter.
Let cook for about 10 minutes, until the mushrooms have given off all their liquid and it's evaporated. Transfer to a plate and set aside.
Add the other tablespoon of butter to the pot.
Once melted, add the leeks and cook, stirring occasionally for about 10 minutes until softened and starting to brown.
Add the garlic and buckwheat groats to the pot. Toss and let cook so the garlic becomes fragrant and the buckwheat "toasts", about 1-2 miinutes.
Add the wine to the pot, stir and let cook until it's completely absorbed.
At this point, ladle in the vegetable broth about 1/2 cup at a time, keeping the mixture at a low simmer.
As the liquid starts to get absorbed by the buckwheat, add a bit more to the pot until you've used all 3 cups and it's been absorbed fully by the buckwheat. The buckwheat should be tender at this point.
Turn off the heat, add the mushrooms and parmesan to the pot and stir to combine.