Finely dice the onion. Cut the courgette and broccoli stem into roughly 1cm cubes. Roughly chop the spinach.
In a large skillet, heat the oil and butter, add the chopped onion, broccoli stem, and courgette. Add salt and sauté for about 5 minutes, until the vegetables start to soften.
Add the buckwheat and stir well to coat it evenly in the oil.
Get your stock ready next to the skillet. Add one ladle of broth at a time, stirring and letting the liquid absorb before adding more. Repeat until the buckwheat is tender (about 15 minutes).
Finally, stir in the grated Parmesan and chopped spinach. Let everything combine well and serve warm.