Drain the canned black olives in a fine-mesh strainer and rinse briefly under cold water. Shake off excess water and pat dry with paper towels. If olives are whole and you prefer, halve or lightly crush them with the flat side of a knife for more surface area to absorb flavors.

Make marinade

In a small mixing bowl, whisk together the extra-virgin olive oil, fresh lemon juice, thinly sliced garlic, crushed red pepper flakes, freshly ground black pepper, and coarse sea salt until combined. If using a fresh rosemary sprig, strip the leaves and finely chop them; if using dried rosemary, add it directly and stir to incorporate.

Combine and rest

Place the drained olives in a shallow serving bowl or jar. Pour the marinade over the olives, tossing gently with a spoon so the garlic and herbs distribute evenly. Cover the bowl or seal the jar and refrigerate for at least 1 hour to allow flavors to meld (preferably 4–24 hours for best flavor). If you halved or crushed the olives, 1 hour will be sufficient; whole olives benefit from a longer rest.

Finish and adjust seasoning

After marinating, taste an olive and adjust seasoning as needed—add a pinch more salt, a squeeze of lemon juice for brightness, or a drizzle more olive oil for silkiness. If the garlic flavor is very strong after resting, remove the garlic slices or leave them for serving depending on preference.

Optional garnishes and serving

Before serving, sprinkle chopped fresh parsley and lemon zest over the marinated olives. Serve at room temperature alongside toasted baguette slices or crackers. Leftovers keep in an airtight container in the refrigerator for up to 5 days; bring back to room temperature before serving to restore aroma and texture.