Preheat the broiler on high.
Place the chicken cubes on a tray or in a shallow bowl and season both sides with salt and black pepper.
Heat a 10-inch (26 cm) ovenproof skillet over medium-high heat. Add olive oil and ⅓ of the chicken cubes making sure to not crowd the pan. Sauté the chicken 1-2 minutes per side until lightly brown. Transfer onto a plate and do the same with the remaining chicken.
Lower the heat to medium-low and cook the onions and garlic for about 8-10 minutes or until slightly golden, translucent, and soft.
Add tomatoes, water, sugar, oregano, crushed red pepper flakes, black pepper, and a little salt. Take care not to add too much since the feta cheese that will be added in the end will be salty.
Cook the sauce until it reduces and thickens.
Put all of the chicken into the pan, top with crumbled feta cheese, and bring to a boil.
Transfer the pan (if it is oven-proof) to the oven on the center rack and cook for about 5-8 minutes or until the cheese melts, the sauce is bubbly and slightly charred.
Garnish with scallion slices and serve.
Serve with hearty, toasted bread or over pasta.
