Preheat oven to 218°C (425°F). Line a large sheet pan with aluminum foil and set aside.
Cut potatoes into bite-size (about an inch) pieces, and add to a big mixing bowl.
Cut red onion into chunks, and add to the bowl with the potatoes.
Peel garlic cloves, slice thinly, and add to bowl with potatoes and red onion.
Cut lemon into wedges and set aside.
Cut chicken breasts into strips and add to bowl with potatoes, red onion, and garlic.
Squeeze the lemon wedges into the bowl, and then add the squeezed wedges to the bowl with the chicken, potatoes, onion, and garlic.
Add the oregano, salt, pepper, and olive oil to the bowl. Stir it all together until everything is coated in the seasonings, and then add to a sheet pan lined with aluminum foil.
Place sheet pan in the oven and cook for 30 minutes.
While the chicken and potatoes are cooking, make the Honey Lemon Tahini Sauce.
In a mixing bowl, combine tahini, yogurt, garlic, the juice from one lemon, and honey; stir until completely mixed.
Add water, 1 spoon at a time, mixing after each addition. You’re adding water until the consistency of the tahini sauce becomes pourable, like thick salad dressing. You may not need all of the water listed in the ingredients, and sometimes you’ll need more, so by adding a bit at a time, you won’t make it too runny.
At 30 minutes, remove the chicken from the oven and add the crumbled feta and crushed rosemary. Put back in the oven and cook for a further 15 minutes.
Remove from oven and let sit for about 5 minutes before serving. Sprinkle with chopped parsley if using.
Serve Sheet Pan Greek Chicken and Potatoes with a generous drizzle of the Honey Lemon Tahini Sauce, and a nice Greek Salad on the side.