Heat the olive oil in a large pot or deep skillet over medium heat.
When the oil is hot, but nowhere near smoking, add the onions.
Cook them gently for 5-7 minutes until they soften and start to caramelize around the edges, stirring frequently.
Add the garlic and cook for 1 minute, stirring frequently.
Carefully add the tomatoes to the pan and give everything a good stir.
Add the olives, oregano and crushed red pepper and stir well to combine the ingredients.
When the sauce starts to boil, lower the heat to medium-low to low, cover and cook the tomato and olive sauce for 30-40 minutes, stirring occasionally.
Keep the sauce at a simmer. Lower the heat a little if it starts boiling too vigorously.