Bring a pot of water to a boil and cook the buckwheat according to package instructions. Drain if needed and set aside.
Meanwhile, heat olive oil in a large pan over medium heat. Add the diced onion and cook for a few minutes until softened. Add the sliced garlic and cook for another minute.
Cut the broccoli into small florets and dice the zucchini. Add them to the pan along with the spinach and peas. Cook for a few minutes.
Pour in the soy sauce and about 1/2 cup of water. Cover and let the vegetables simmer for about 10 minutes, until tender.
Add the cooked buckwheat to the pan, pour in the cream, and stir well. Cook for a few more minutes until everything is heated through and slightly creamy.
Season with salt and pepper to taste.
Serve topped with crumbled feta cheese.