In a medium saucepan over medium heat, combine olive oil and onion. Cook until softened, 3–4 minutes.
Add anchovies, using a spoon to smooth them into a paste.
Add garlic and sauté 30 seconds.
Add tomatoes with juices and break them into pieces.
Add Kalamata olives, capers, herbs, and red pepper flakes. Simmer 30 minutes, stirring occasionally. Season to taste with salt.