Heat the olive oil and butter over medium heat in a skillet. Add the onion and cook until softened, about 3-5 minutes. Add the garlic and cook 30 seconds more, until fragrant.
Add the mushrooms, thyme, salt, and pepper. Cook for about 3-4 minutes. Add the buckwheat and cook for 1-2 minutes more.
Start adding the broth, ½ cup at a time, stirring. Only add the next portion of the broth when the previous has absorbed by buckwheat. Cook until the buckwheat is al dente, about 12-15 minutes.
Pour in the wine and increase the heat. Cook for 2-3 minutes, letting the alcohol evaporate. Add the heavy cream and Parmesan cheese. Cook, stirring, until the cheese has melted. Serve garnished with the reserved Parmesan cheese.
