Preheat the oven to 400°F (200°C). Cut cauliflower into bite-sized florets if not already done. Peel and dice Yukon Gold potatoes into 1/2-inch cubes so they roast and cook in roughly the same time as the cauliflower. Dice red bell pepper and finely chop the onion. Mince the garlic and chop the parsley; set aside.

Par-cook potatoes

Place diced potatoes in a medium pot and cover with cold water. Add 1/2 tsp kosher salt. Bring to a boil, then reduce to a simmer and cook 6–7 minutes until just tender but not falling apart. Drain well and return to the pot to steam-dry for 1 minute.

Roast cauliflower, potatoes & pepper

On a large rimmed baking sheet, toss the par-cooked potatoes, cauliflower florets, and diced red bell pepper with 1 tablespoon olive oil, 1/2 tsp kosher salt, 1/4 tsp black pepper, smoked paprika, and ground cumin. Spread in a single layer. Roast in the preheated oven for 18–22 minutes, turning once halfway through, until cauliflower is tender and edges are golden and potatoes are fork-tender.

Dairy & Cheese

Make pepper-jack cheese sauce

While vegetables roast, melt the butter in a medium skillet over medium heat. Add the chopped onion and cook 5–6 minutes until softened and translucent. Add the minced garlic and cook 30 seconds until fragrant. Sprinkle the flour over the onion-garlic mixture and stir for 1 minute to cook the raw flour taste. Slowly whisk in the milk and the remaining 1 tablespoon olive oil (if needed to loosen). Cook, whisking, until the mixture thickens and bubbles, about 2–3 minutes. Reduce heat to low and stir in the shredded pepper jack cheese a handful at a time until fully melted and smooth. If the sauce becomes too thick, whisk in up to 1–2 tablespoons additional milk. Taste and adjust seasoning with 1 tsp kosher salt (or to taste), 1/4 tsp black pepper, and red pepper flakes if using.

Assemble & Finish

Combine roasted vegetables and sauce

Remove the roasted vegetables from the oven. Transfer them to a large mixing bowl. Pour about three quarters of the warm pepper-jack sauce over the vegetables and gently fold to coat. Reserve the remaining sauce for topping.

Bake briefly to meld flavors (optional)

If you prefer a bubbled finish, transfer the sauced vegetables to a greased 9x9-inch baking dish, spread evenly, and sprinkle the remaining shredded pepper jack over the top. Return to the oven at 400°F (200°C) for 8–10 minutes until cheese is bubbly and lightly golden.

Finish and garnish

Remove from oven and let rest 3–4 minutes. Taste and adjust seasoning with additional salt and pepper if needed. Sprinkle chopped fresh parsley over the dish before serving.