Place carrots, baby potatoes, broccoli florets, red bell pepper, zucchini, and red onion in a very large bowl or on the lined baking sheet for tossing. Make sure pieces are sized so they cook evenly (roughly similar thickness).

Dress vegetables

In a small bowl whisk together olive oil, melted butter (if using), minced garlic, thyme leaves, lemon zest, kosher salt and black pepper. Pour the oil-herb mixture over the vegetables and toss thoroughly so all pieces are lightly coated.

Arrange and roast

Spread the vegetables in a single layer on the prepared baking sheet with space between pieces (use two sheets if crowded). Roast in the preheated oven for 20 minutes. After 20 minutes, stir or flip the vegetables and rotate the sheet. Continue roasting 8–12 more minutes, until potatoes are tender and edges are golden-brown and caramelized. Total roasting time should be 28–32 minutes depending on vegetable size.

Finish with lemon

Remove the pan from the oven and immediately squeeze the lemon juice over the hot vegetables and toss gently to coat. Taste and adjust seasoning with additional salt and pepper if needed.

Finishing & Garnish

Transfer the roasted vegetables to a warm serving dish. Sprinkle chopped parsley over top, then add shaved Parmesan and toasted nuts if using. Serve immediately.