Kalamata (also spelled Calamata) olives are purplish-brownish olives from Greece that are salty and pungent.
Castelvetrano (sometimes also known as Castelnuovo) olives are large, bright green, meaty, almost buttery olives from Sicily, with a flavor that is milder than your typical olive.
Once you have grated the onion into a bowl, transfer it into a fine-mesh sieve. Set the sieve over the bowl and let it drain for 15 minutes, or until most of its liquid has been released and it no longer drips.
To grill peppers, heat a grill to high heat. Rub each pepper with a little olive oil and place on the hottest part of the grill. Let the peppers grill, turning occasionally with tongs, until the skin blackens all around. If you prefer to roast them the night before, preheat the oven to 350°F. Rub each pepper lightly with a little olive oil, place on a baking sheet, and roast for 50 minutes to 1 hour, until tender but not brown. If you have a gas stovetop, you can put the oiled peppers right on the grates of the hottest, highest flame you have and allow them to blacken all round, turning them occasionally with large tongs and working in batches as necessary. Whichever way you cook them, place the warm peppers into a bowl and cover with plastic wrap. Cool to room temperature. Peel off the skin. Cut open and remove the stems and seeds. It’s fine if there is a little burned skin left on the peppers. Do not rinse them. Proceed as your recipe instructs.