Heat olive oil in a large pan over medium-high heat until hot. Cook the chicken breasts for 12-15 minutes (depending on thickness) until the internal temperature reaches 74°C (165°F). Transfer to a plate.
In the same pan, melt the herb butter over medium heat. Add the chopped onion and diced bell pepper. Cook for 3-4 minutes, stirring occasionally, until the onion is translucent and the pepper is soft.
Add the canned tomatoes and break them up with a fork. Simmer for 2 minutes.
Pour in the milk or cream and add the cheese. Season with basil, oregano, salt, and pepper. Simmer for 3-5 minutes, stirring occasionally, until the cheese melts completely.
Prepare the couscous according to packet instructions.
Serve the chicken with the creamy tomato sauce spooned over the couscous.