Preheat your oven to 200°C | 400° F. Season chicken thighs with salt, pepper and dried thyme.: As you preheat, breathe in the warm, anticipatory scent of an oven coming to life, it signals that you are ready to transform the ingredients. Seasoning at this point primes the surface for browning, and the dried thyme will toast slightly during searing releasing subtle, savory notes. Listen for the tiny hiss when seasoned meat hits a hot pan, that sharp sound tells you the Maillard reaction is starting. A common mistake here is seasoning too early and letting moisture pool on the skin, which inhibits crisping; season just before cooking for the best crust.
Heat a large (34 cm or 13-14 inch) cast iron skillet (or heavy based oven-proof pan), over medium-high heat. When the skillet (or pan) is hot, add the oil. Sear the thighs, skin-side down, until skin is golden and crisp (about 4-5 minutes). Flip chicken and sear on the other side for a further 5 minutes.: You will smell a rich, toasty note as the pan and oil reach temperature, creating a sizzling symphony when the chicken hits the surface. Visual cues are key, watch for an even deep golden color on the skin, that is when the fat has rendered nicely. The sizzling should be steady not furious; if the oil smokes, lower the heat. One trap is moving the pieces too soon, which tears the skin and prevents good browning; give each piece the time it needs to release naturally.
Melt the butter in the pan. Add the garlic until fragrant (about 30 seconds). Pour in the lemon juice, and sprinkle over fresh thyme leaves from 2 sprigs. Place another 2 sprigs around the chicken.: The moment the butter melts you will notice a softer, almost caramel like aroma blending with the seared notes. Adding the garlic briefly softens and sweetens it, releasing its essential oils into the fat. When the lemon juice hits the warm fat there is a lively hiss and a bright citrus perfume that cuts through richness. Strip a few fresh thyme leaves to scatter, their green fragrance lifts the whole pan. Avoid leaving the garlic in at high heat long enough to brown deeply, as that can introduce bitterness rather than sweet aromatics.
Transfer skillet (or pan), to the oven and cook for 25-30 minutes, or until completely cooked through. Add the asparagus spears to the pan during the last 12 minutes of cook time.: Moving the pan to the oven creates an even, enveloping heat that finishes the interior without drying it. As the chicken roasts, the pan juices will gently bubble and concentrate, delivering a glossy coating. Slide the trimmed asparagus into the pan with 12 minutes remaining so each spear softens but keeps a snap and vibrant color. If you add the asparagus too early, it will become limp and lose that fresh bite which balances the meal.
Garnish with lemon slices and leaves from the remaining sprig of thyme. Serve immediately.: When you pull the pan, the room fills with a layered aroma of roasted meat, bright citrus, and herb oil, which is part of the joy of serving straight from the pan. Scatter the thin lemon slices and fresh thyme leaves to add bursts of freshness and visual contrast. Serve right away to capture the juicy textures and warm buttery sauce; letting it sit too long will cool the pan juices and change the mouthfeel. A common oversight is not letting the meat rest briefly if very hot; a short 3 to 5 minute rest helps the juices redistribute and keeps each bite succulent.