Preheat oven to 425°F (220°C). Trim beet greens close to the root and peel the beets with a vegetable peeler. Cut beets into 1/2- to 3/4-inch cubes so they roast evenly with the other vegetables.

To ensure beets roast through at the same time as carrots and cauliflower, par-cook them: place beet cubes in a small pot, cover with water, bring to a simmer, and cook 6–8 minutes until just fork-tender but not falling apart. Drain and let steam-dry in the colander for 2–3 minutes.

Prepare Vegetables & Seasoning

While beets par-cook, cut cauliflower into bite-sized florets, trim and halve (or leave whole small) rainbow carrots. If using red onion, cut into 8 wedges. Mince garlic and strip thyme leaves from stems.

In a large mixing bowl combine the olive oil, minced garlic, thyme leaves, sea salt, black pepper, smoked paprika (if using) and lemon juice. Whisk briefly to blend.

Add the par-cooked beets, cauliflower florets, carrots, and onion wedges to the bowl. Toss thoroughly so all pieces are evenly coated with the seasoned oil. Arrange the vegetables in a single layer on one or two rimmed baking sheets (use parchment or lightly oiled sheets). Avoid overcrowding to promote browning.

Roasting

Roast in the preheated 425°F (220°C) oven for 25–30 minutes, rotating the pans and stirring the vegetables once halfway through (about 12–15 minutes). Roast until edges are caramelized and vegetables are tender when pierced with a fork. If some pieces are browning too fast, reduce oven to 400°F (200°C) and continue roasting until done.

Finish & Serve

Remove the vegetables from the oven and transfer to a large serving bowl or platter. Taste and adjust seasoning with a pinch more salt or a grind of black pepper if needed.

Sprinkle chopped fresh parsley and crumbled feta or goat cheese over the warm vegetables if using. Serve immediately.