Preheat oven to 375°F (190°C).
Thaw the frozen chopped spinach and squeeze out as much liquid as possible using a clean dish towel, cheesecloth, or several layers of paper towels. Set spinach aside.
If the 6 oz cream cheese is not softened, let it sit at room temperature until soft or microwave briefly (10–15 seconds) until spreadable.
In a large bowl, beat the softened cream cheese and 1/4 cup sour cream together until smooth.
Stir the entire 10 oz can of Rotel (including juices), the squeezed spinach, the drained 2.25 oz diced black olives, and 1 cup of the shredded colby jack cheese into the cream cheese mixture until evenly combined.
Arrange the 2 lb boneless skinless chicken breasts in a single layer in a large baking dish. Pat the chicken dry with paper towels; if breasts are uneven in thickness, pound gently to even thickness for more even cooking.
Spread the spinach–cheese mixture evenly over the top of each chicken breast.
Sprinkle the remaining 1/2 cup shredded colby jack cheese evenly over the spinach mixture.
Bake uncovered for 30–40 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven, let rest a few minutes, then garnish with chopped green onions and chopped cilantro. Serve hot.