Preheat and prep pans: Heat the oven to 425°F (220°C). Line 2 large sheet pans with parchment for easy cleanup. Use two pans so the veggies have space to brown.
Cut for even cooking: Keep pieces around 1 inch so they cook at the same pace.
Halve any oversized Brussels sprouts, and keep onion wedges intact so they don’t burn.
Season generously: In a large bowl, toss veggies with olive oil, salt, pepper, garlic, thyme, rosemary, and red pepper flakes if using. Everything should be lightly coated but not greasy.
Spread it out: Divide the veggies between the pans. Do not crowd or they will steam instead of roast. Flip Brussels sprouts cut-side down for better sear.
Roast, then rotate: Roast for 15 minutes, then rotate pans (top to bottom, back to front).
Stir or flip the veggies so new sides touch the pan.
Finish roasting: Roast another 15–20 minutes until the edges are deeply golden and the centers are tender. Total time is about 30–35 minutes.
Add the glossy finish: While hot, toss the veggies with maple syrup and lemon juice. This brightens the flavor and adds a light sheen without making them sweet.
Top and serve: Sprinkle with parsley and any optional add-ins like pomegranate arils or toasted nuts.
Taste and adjust salt and lemon if needed.