Preheat and prep. Preheat the oven to 200°C / 400°F (convection 190°C / 375°F). Pat the pork completely dry with paper towels—this removes surface moisture and ensures a proper sear. Toss the cubed butternut with 1 tbsp olive oil, 1/2 tsp salt, 1/4 tsp pepper and half the chopped rosemary; spread in a single layer on a rimmed baking sheet so the pieces roast rather than steam.

Season the pork. Trim any silver skin from the tenderloin if present. Rub the pork all over with the remaining 1 tbsp olive oil, 1 tsp kosher salt and 1/4 tsp black pepper, then press the remaining rosemary and 1 minced garlic clove onto the surface. The oil helps the seasoning adhere and the rosemary infuses aromatics into the exterior during searing.

Sear the pork to develop flavor and color. Heat a large ovenproof skillet (cast-iron preferred) over medium-high heat until shimmering. Place the pork in the dry pan and sear without moving for 2–3 minutes until a deep golden-brown crust forms; rotate and brown the other sides, about 2 more minutes. Searing creates the Maillard crust that delivers concentrated savory flavor and an attractive presentation.

Start roasting. Transfer the seared pork to one side of the baking sheet with the butternut (or leave in the skillet if there’s space). Scatter the remaining minced garlic over the squash so it roasts and sweetens rather than burning. Roast in the preheated oven for 18–22 minutes, until an instant-read thermometer inserted into the thickest part of the tenderloin reads 60°C / 140°F for medium (or 63–65°C / 145–150°F for medium-well). Because tenderloin is lean and cooks quickly, the oven finish ensures even cooking through the center without overcooking the outside.

Check and crisp. At 12–15 minutes, check the squash: if edges are golden, toss the cubes gently to promote even browning. If some cubes are coloring faster, shift them away from the pork. Continue cooking until squash is tender and edges caramelized—caramelization concentrates sweetness and adds depth to the dish.

Rest the pork. Transfer the pork to a cutting board and tent loosely with foil for 8–10 minutes. Resting lets the muscle fibers relax and reabsorb juices, producing a juicier sliced tenderloin. Keep the roasted squash on the sheet to finish while the pork rests (carryover heat will keep it warm).

Quick pan sauce (optional but recommended). If you used an ovenproof skillet to sear the pork, place it over medium heat, add 1–2 tablespoons water and scrape up browned bits with a wooden spoon—those fond bits are flavor gold. Reduce a minute or two to concentrate, then finish with a small knob of the reserved butter (if desired) and a turn of pepper. Spoon the glossy juices over the sliced pork for an extra layer of savory flavor.

Sauté the spinach. While the pork rests, heat a large sauté pan over medium heat and add the 1 tbsp unsalted butter. When it foams, add the remaining 2 minced garlic cloves and shake the pan for 20–30 seconds—this softens the garlic and flavors the fat without burning it. Add the spinach in batches, tossing and stirring until just wilted, about 2–3 minutes total. Season with a pinch of salt and a crack of black pepper. Quick cooking keeps the spinach bright green and prevents it from becoming soggy.

Slice and serve. Slice the pork into 1.5–2 cm (1/2–3/4") medallions on the bias to show the tender interior. Plate a bed of sautéed spinach, arrange roasted butternut cubes beside it, and fan the pork medallions on top. Spoon any pan juices over the pork and finish with a light sprinkle of flaky sea salt and freshly cracked pepper. The contrast of textures—silky spinach, crisped-edged squash and tender pork—makes each bite interesting.

Final garnish idea (optional). For brightness, shave a little lemon zest over the spinach or sprinkle chopped fresh parsley. A few toasted pine nuts or a drizzle of aged balsamic also pairs beautifully with the roasted squash for an extra touch of elegance.