Place a skillet over medium heat and pour in the olive oil. Once hot, saute the chicken breasts for 8 minutes, flipping once, until no longer pink inside.
Meanwhile, stir together the biscuit mix with water, cheddar cheese, and green onion in a medium mixing bowl. Set aside.
Spray the inside of a 9-inch x 13-inch baking dish with non-stick cooking spray. Spread the marinara sauce in the bottom of the dish, lay the chicken over the sauce, and then spoon the prepared biscuit mix over the chicken breasts. Use the back of a spoon to spread the biscuits to cover the chicken completely.
Bake at 400 degrees F for 25 minutes, until the top of the biscuit layer is golden brown. Remove from the oven.
Combine the melted butter and seasoning packet from the biscuit mix box in a small bowl. Then brush the butter over the top of the casserole. Allow to cool for 5-10 minutes before serving warm.