Preheat and Prep - Preheat oven to 425°F (220°C). Line a large rimmed baking sheet with parchment paper.
Prepare Vegetables - Wash and cut baby potatoes in half. Cut carrots into 1-inch pieces, bell peppers into 1.5-inch pieces, and onion into wedges.
Season Root Vegetables - In a large bowl, toss potatoes and carrots with 2 tablespoons olive oil, garlic powder, onion powder, smoked paprika, thyme, salt, and pepper.
Start Roasting - Arrange seasoned potatoes and carrots on the prepared baking sheet. Roast for 15 minutes.
Add Remaining Ingredients - Score kielbasa diagonally every inch. Add kielbasa, bell peppers, and onions to the pan. Drizzle with remaining olive oil.
Continue Cooking - Roast for an additional 20-25 minutes, until vegetables are tender and kielbasa is golden brown.
Final Browning - Optional: Increase temperature to 450°F for the last 5 minutes for extra caramelization.
Rest and Serve - Let rest for 5 minutes before serving. Garnish with fresh herbs if desired.