In a large skillet, heat the oil over a medium-high flame. When almost smoking, add the onion and garlic and sauté for 3 minutes. Reduce the heat to medium and add the olives, capers, 1/2 teaspoon of salt, and red pepper flakes, and sauté for 1 minute. Add the tomatoes and simmer until reduced slightly, about 20 minutes. Season the sauce with more salt to taste. (The sauce can be made 1 day ahead. Cool, then cover and refrigerate. Rewarm over medium heat before using.)