Trim chicken breasts and season on both sides with salt, pepper and garlic powder.
Heat a large skillet over medium-high heat, add olive oil then place chicken in skillet. Cook 4–5 minutes per side or until cooked through. Remove chicken to a serving platter and cover with foil to keep warm.
Wipe out skillet and reduce heat to medium. Add butter, allow it to melt then add flour and stir to combine. Cook flour, whisking constantly for approximately 1 minute until it just begins to turn light brown. Whisk in milk and allow the mixture to come to a bubble.
Stir in oregano, garlic powder and lemon zest then reduce heat to low and simmer 5–6 minutes, stirring occasionally until slightly thickened. Add feta crumbles, stir until cheese is mostly melted then remove from heat.
Season sauce to taste with salt and pepper and adjust consistency of sauce with a bit of extra milk if necessary. Pour sauce over chicken breasts and sprinkle with diced tomatoes and sliced green onions. Serve immediately and enjoy.
One Last Thing
Serve over angel hair or orzo pasta—there’s plenty of sauce to go around.