Preheat the oven to 400℉. Line a large baking sheet with parchment paper or a silicone baking mat.
Place potatoes, onion, garlic, and cabbage on the baking sheet, being careful not to separate the leaves of each cabbage wedges. Drizzle with oil. Gently toss the potatoes, onions, and garlic with oil and carefully rub the oil over the surface of the cabbage wedges.
Turn potatoes cut-side down. Arrange kielbasa over potatoes and onions. Sprinkle all with salt and pepper and nestle thyme sprigs into the veggies. If using dried thyme, sprinkle over the veggies along with the salt and pepper.
Place pan in preheated oven and bake 45-55 minutes, rotating pan halfway through the baking time, or until potatoes are tender and golden brown on their undersides.
Remove from the oven, serve with mustard if desired.
