Preheat oven to 220 C / 200 fan. Line a baking tray with greaseproof paper.: You will notice the oven filling with dry hot air that encourages Maillard browning, and the paper prevents sticking while making cleanup easier. This high heat is important to get deep color on the cauliflower , which provides caramelized sweetness, and it also crisps any exposed surfaces. A common mistake is using a lower temperature to "be safe," which results in pale, steamed vegetables instead of roasted ones. Make sure the oven truly reaches the temperature before you slide the tray in, and arrange racks so the tray sits in the middle for even heat circulation.
Mix the cauliflower florets, onion, celery, and garlic. Toss with 2 tablespoon the olive oil, ghee or butter and seasoning.: As you toss, the oil glazes each piece and helps heat transfer, so the cauliflower and diced onion begin to shimmer and release faint aromas of sweet onion and raw garlic . This coating encourages crisp edges, which are essential for texture contrast. If you skip thorough tossing, pockets of dry vegetable will underbrown, and the seasoning will be uneven. Use your hands or a large spoon to ensure even coverage, and taste a small cooked piece later to adjust seasoning if needed.
Spread in a single layer and roast for 15 minutes. Add the herbs, pistachios and chestnuts. Stir to combine and roast for a further 15 minutes until the cauliflower is browned and the onion caramelized.: The first roast blast softens interiors while the second phase deepens color and concentrates sugars, releasing a sweet, nutty smell. When you stir in the rosemary , sage , thyme , pistachios , and chestnuts , their oils bloom and perfume the pan. Look for golden edges and audible faint crackles from the nuts. A common issue is stirring too often, which prevents the surface from browning; instead, let the vegetables sit undisturbed for the roasting intervals so caramelization can occur.
Meanwhile add the remaining oil, ghee or butter to a pan. You may choose to omit this and dry fry depending on how fatty your sausages are. Remove the sausage meat from the skin using a sharp knife to slice the skin. Crumble the sausage meat into a non stick frying pan and break with a wooden spatula. Cook for approximately 7 minutes on a medium heat until it is browned.: As the sausages hit the pan, they will sizzle and release flavorful browned bits that are essential for umami. Browning creates tiny caramelized pieces that deepen the overall taste and render fat to coat the other ingredients. Watch for even browning and break up clumps so you get small, crisped bits rather than large lumps. A frequent error is to cook on too high heat, which can burn the outside before the interior is cooked; keep the temperature at medium and stir occasionally to create consistent color and texture.
Combine all ingredients in a large roasting pan and cook in the oven for about a further 10 minutes, or until the sausage is slightly crisp on top. Option to scatter with fresh thyme to serve.: At this final stage the oven heat helps the flavors marry and any rendered fat from the sausages lightly crisps the surface, adding textural contrast. You will smell a heady mix of herbs and roasted nuts, and the top will take on a slight crust, which I find irresistible. If you skip this finishing bake, the flavors may remain separate and the dish will lack that cohesive roasted aroma. Keep an eye on the top so you hit a golden finish without overbrowning, and add fresh thyme at the end for a bright herbal lift.