Preheat large (12 inch) deep skillet on low-medium heat and swirl oil to coat. Add onion and bell pepper; saute for 3-4 minutes, stirring occasionally.
Add portobello mushrooms and saute for another 3-4 minutes, stirring occasionally.
Add chicken breasts and cook for 3-4 minutes, stirring often, until each piece of chicken is separate and opaque. No need for chicken to be browned.
Add tomatoes and their juices, olives and their brine, sugar, red pepper flakes, salt and pepper. Stir, cover and bring to a boil. Then reduce heat to low and simmer for 20 minutes.
While skillet is cooking, cook baby potatoes. In a large pot, add potatoes and enough cold water to cover them plus a pinch of salt. Cover, bring to a boil, then reduce heat to low and cook for 15-20 minutes or until potatoes are soft when pierced with a fork. Drain and set aside.
Turn off heat, add garlic, stir and let skillet stand for 5 minutes. Garnish with parsley and serve over baby potatoes.
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Notes
I use San Marzano whole tomatoes because they and their juice taste the best. I prefer to cut them instead of using canned diced tomatoes but in a pinch you can use the latter as well.
Nutritional information is without baby potatoes.
Store: Refrigerate in an airtight container for up to 3 – 4 days.
Freeze: Freeze leftovers without baby potatoes in an airtight container for up to 3 months. Thaw in the fridge overnight or on a counter for a few hours.
To reheat: Simmer on low heat in a skillet for 5 minutes covered.