Prepare the pork: Pat the 250 g pork tenderloin dry with paper towels (removing surface moisture helps get a better sear). Trim any silver skin and excess fat so seasoning adheres evenly. Season all over with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper. Press the chopped rosemary and thyme onto the surface of the meat with your hands, then rub the minced garlic into the herb crust so it adheres. Let the pork sit at room temperature for 15 minutes while you preheat the oven and prep vegetables — bringing meat closer to room temp shortens cooking time and cooks more evenly.

Preheat oven and prep fennel: Preheat the oven to 400°F (200°C). Toss the sliced fennel (about 200 g) with 1 tbsp of the olive oil, a pinch (about 1/4 tsp) of salt and a few grinds of pepper in a rimmed baking sheet or cast iron skillet. Spread in a single layer so slices can brown. Roasting at high heat caramelizes the fennel's natural sugars and concentrates its anise‑like flavor.

Roast the fennel: Place fennel in the preheated oven and roast for 20 minutes, turning once halfway through so both sides color. Roast until edges are golden-brown and the centers are tender — caramelized edges add sweet, nutty depth that complements the pork.

Sear the pork: While the fennel starts roasting, heat a heavy skillet (cast iron preferred) over medium-high heat until smoking lightly. Add the remaining 1 tbsp olive oil and swirl to coat. Sear the herb‑crusted pork 1.5–2 minutes per side, rotating to get a deep golden-brown crust (including the ends). High-heat searing develops the Maillard crust that delivers flavor and a nice texture contrast to the tender interior.

Roast the pork: Transfer the seared pork to the oven (you can place it on the same sheet as the fennel if there's room — that allows the pork juices to mingle with the fennel) and roast until an instant‑read thermometer inserted into the thickest part reads 145°F (63°C), about 10–12 minutes depending on thickness. 145°F gives a safe, slightly rosy center; remove the pork a few degrees earlier if you prefer rarer, remembering it will rise a few degrees while resting.

Rest the pork: Remove the pork from the oven and transfer to a cutting board. Tent loosely with foil and rest for 5–8 minutes. Resting lets the juices redistribute so they’re not lost when you slice — it also finishes carryover cooking, arriving at the perfect doneness.

Finish the fennel and make pan juices: If fennel needs a final minute of color, return to the oven under broil for 1 minute, watching closely. Once pork is out, place the skillet you used for searing back on medium heat; deglaze with a splash of water (1–2 tbsp) and scrape up any browned bits — those fond bits contain concentrated flavor. Spoon those juices over the roasted fennel for extra savory depth.

Sauté the spinach: Use the warm skillet off the heat or over medium-low heat (the pan retains heat) — add the drained baby spinach (about 2 cups / 60 g) and a small pinch of salt. Toss the spinach briefly with the pan juices and the remaining aromatic oil coating; if it seems dry, add a teaspoon of water. Cook just until wilted, about 1–2 minutes — spinach cooks extremely quickly and prolonged heat makes it lose color and nutrients. Finish with a squeeze of lemon if using to brighten the flavors.

Slice and plate: Slice the rested pork tenderloin into 1/2‑inch medallions against the grain to keep slices tender. Arrange slices over a bed of sautéed spinach with roasted fennel alongside. Spoon any remaining pan juices or fennel drippings over the pork. Finish with a few extra pinches of flaky salt and a grind of black pepper for contrast.

Serve immediately: This dish is best served right away while pork is warm and spinach is bright — the contrast of the herb crust, succulent pork, sweet fennel and the verdant spinach makes every bite balanced and restaurant‑style.