Preheat oven to 425°F (220°C). Line a large rimmed baking sheet with parchment paper or lightly grease it.
In a large bowl, toss the sausage rounds, bell pepper strips, and red onion wedges with olive oil, smoked paprika, fennel seed, garlic powder, salt, and black pepper until evenly coated.
Spread the mixture in a single layer on the prepared baking sheet. Roast for 20 minutes.
Remove from oven, stir the sausage and vegetables, and continue roasting for 10-15 minutes, until the vegetables are tender and lightly charred at the edges and the sausage is browned.
If using, sprinkle fresh thyme over the top. Serve hot, with crusty bread, rice, or in hoagie rolls.