Heat oil in a large pan and add onion. Cook until softened, stir in the garlic. Cook for 1 more min. Mix in the peppers and cook for 5 mins stirring occasionally.
Add the zucchini, marjoram, cover and cook for 7 mins until zucchinis are just tender. Pour the passata and chicken stock into the pan add the chicken and tagliatelle bring to the boil. Reduce the heat and simmer for 5 - 8 mins. Add seasoning and serve.