Remove the rack of pork from the fridge 30–45 minutes before roasting so it isn’t icy cold. Pat it very dry with paper towels. Using a sharp knife, lightly score the fat cap (not the meat) in a crosshatch pattern about 1/2 inch apart to help the fat render and the seasoning penetrate.

Season the rack of pork all over, especially the fat cap and between the bones, with kosher salt and black pepper. If time allows, refrigerate uncovered for up to 24 hours to dry brine for even better flavor and crispier fat.

Preheat the oven to 450°F (230°C). Line a roasting pan or sturdy rimmed baking sheet with foil. Place a wire rack on top if you have one. If not using a rack, arrange the onion rounds in the pan to act as a bed for the pork. Add the rosemary and thyme sprigs to the pan if using. Pour the chicken broth or wine into the bottom of the pan.

In a small bowl, combine the olive oil, minced garlic, chopped rosemary, chopped thyme, Dijon mustard, smoked paprika, onion powder, and crushed red pepper flakes (if using). If you like a subtle sweetness, stir in 1–2 teaspoons brown sugar. Mix until a thick paste forms; add a little more oil if too thick, or a pinch more paprika if too loose.

Place the pork in the prepared pan meaty side up, bones curving downward. Spread the garlic-herb mixture evenly over the top and sides, massaging it into the scored fat cap. Focus on coating the meat and fat cap; the bone side can be more lightly covered.

Roast the pork at 450°F (230°C) for 15–20 minutes to begin browning the herb crust and give the fat cap a head start on crisping. Do not cover the roast.

Without opening the oven if possible, reduce the temperature to 325°F (165°C). Continue roasting until the internal temperature in the thickest part of the meat (not touching bone) reaches 135–140°F (57–60°C). For a 4–6 lb roast, this usually takes a total of about 1 1/4 to 1 3/4 hours. Begin checking the temperature about 45 minutes after reducing the heat.

Transfer the rack of pork to a cutting board. Let it rest uncovered for about 5 minutes, then tent loosely with foil and rest an additional 10–15 minutes (15–20 minutes total). The internal temperature will rise to about 145°F (63°C) as it rests, and the juices will redistribute.

While the pork rests, pour the pan drippings and roasted onions into a small saucepan, discarding excessively burned bits if there are any. Skim off excess fat if needed. Bring to a simmer over medium heat and reduce slightly. If desired, add a splash more broth or wine. Taste and adjust seasoning with salt and pepper, then whisk in 1–2 tablespoons cold butter off the heat for a richer sauce.

Turn the rested rack bone-side up so you can see the bones clearly. Use a sharp carving knife to cut straight down between the bones into thick chops. Serve 1–2 ribs per person, spooning some pan sauce over each portion. Pair with roasted potatoes, green beans, or your favorite sides.