Preheat the oven to 400°F. In a large mixing bowl, toss the zucchini, bell peppers, and red onion with 2 Tbsp. of olive oil, salt, and pepper until evenly coated. Spread the vegetables on a baking sheet in a single layer. Roast for 20-25 minutes, or until tender and lightly caramelized, stirring halfway through.
While the vegetables are roasting, bring 2 cups of water to a boil in a medium saucepan. Add the rinsed buckwheat groats, reduce heat to low, cover, and simmer for 10-12 minutes, or until the water is absorbed and the buckwheat is tender. Remove from heat and let it sit, covered, for 5 minutes. Fluff with a fork and allow to cool slightly.
Prepare the dressing by whisking together the balsamic vinegar, extra virgin olive oil, honey, Dijon mustard, salt, and pepper in a small bowl.
In a large salad bowl, combine the cooked buckwheat, roasted vegetables, chopped parsley, and crumbled feta cheese. Drizzle with the dressing and toss gently to combine.
Taste and adjust the seasoning with additional salt and pepper if needed. Serve warm or at room temperature.