Place the Castelvetrano olives in a fine-mesh colander and rinse under cool running water for 20–30 seconds to remove excess brine. Spread the olives on a clean kitchen towel or paper towels and pat dry thoroughly. Drain any remaining liquid from the olive container.

Prepare Aromatics

Zest the lemon using a fine grater, then cut the lemon in half and squeeze 1 tablespoon of fresh lemon juice (reserve remaining lemon for garnish). Smash the two garlic cloves lightly with the flat side of a chef's knife and peel them. Strip the rosemary leaves from the sprig if preferred (you can also leave the sprig whole for appearance) and rinse the thyme sprigs.

Warm the Oil (optional)

In a small saucepan over low heat, warm the olive oil just until it begins to shimmer, about 1–2 minutes. Do not let it smoke. Warming the oil briefly helps release the aromatics' flavors more quickly, but you can skip this step and use oil at room temperature.

Combine Ingredients

In a mixing bowl or jar with a tight-fitting lid, combine the dried Castelvetrano olives, warmed (or room-temperature) olive oil, lemon zest, 1 tablespoon fresh lemon juice, smashed garlic cloves, rosemary sprig (or leaves), thyme sprigs, crushed red pepper flakes, whole black peppercorns, coarse sea salt, and fennel seeds if using. Stir or shake gently to coat the olives evenly in oil and aromatics.

Marinate

Cover the bowl or seal the jar and refrigerate for at least 6 hours, preferably overnight, to allow flavors to meld. For best flavor, let the olives come to room temperature for 15–30 minutes before serving so the oil softens and the aromatics are more pronounced.

Finish and Serve

Transfer the marinated olives to a shallow serving bowl. Remove and discard the rosemary and thyme stems if left whole and the smashed garlic if desired (you can leave garlic for extra flavor). Garnish with the thin lemon slices. Serve with crusty bread, crostini, or as part of an antipasti platter. Store leftovers in the refrigerator, submerged in the oil, for up to 2 weeks; bring to room temperature before serving.