Heat olive oil in a skillet over medium heat. Cook and stir onion, bell pepper, and garlic in the hot oil until the onion is translucent, 5 to 7 minutes. Dust the onion mixture with cayenne pepper and transfer to a bowl; set aside to cool.
Toss avocado, tomato sauce, lemon juice, olive oil, coriander, and cilantro together in a large bowl. Add onion mixture and gently mix to combine.
Cover bowl with plastic wrap; refrigerate until cool, at least 1 hour.