1. Preheat your oven to 400 F (200 C). Line a large baking sheet with parchment paper for easy cleanup.

2. Peel the outer layers from the yellow onions. Cut each onion in half through the root end, then slice them into 1/2-inch thick wedges. Try to keep the root end intact on each wedge to help the layers stay together during roasting.

3. Place the sliced onions in a large mixing bowl. Drizzle them with olive oil, tossing gently to ensure all the onion pieces are lightly coated.

4. In a small bowl, combine the smoked paprika, garlic powder, onion powder, dried oregano, cayenne pepper, black pepper, salt, and red chili flakes. Sprinkle this spice mixture over the oiled onions in the large bowl. Toss thoroughly with your hands or a spoon until every piece of onion is evenly coated with the vibrant spices.

5. Spread the seasoned onions in a single layer on the prepared baking sheet. Avoid overcrowding the pan; use two sheet pans if necessary.

6. Bake for 20-25 minutes, or until the onions are tender, slightly caramelized at the edges, and fragrant. Halfway through the cooking time (around 10-12 minutes), give the onions a gentle stir or flip them with a spatula to ensure even browning.

7. Once cooked, remove the sheet pan from the oven. If desired, sprinkle with fresh chopped parsley for a pop of color and freshness. Serve immediately as a hot side dish.