Preheat oven to 350°F. Lightly spray a 9 x 13-inch baking dish with cooking spray.
Open the 13.5-ounce can of coconut milk. Measure out 1 cup of the coconut milk and set it aside for cooking the rice; reserve the remaining coconut milk from the can for the casserole.
Cook 1 1/2 cups uncooked rice according to the package directions, substituting the 1 cup of coconut milk you measured in step 2 for one cup of the water called for on the rice package. Follow the package for any additional water and cooking time.
While the rice cooks, drain the 20-ounce can of pineapple chunks and cut into smaller pieces if desired. Drain and dice the 8-ounce can of water chestnuts (if not already diced). Thinly slice the 6 green onions. Make sure the 3 cups cooked chicken is diced or shredded.
In a large bowl (or directly in the prepared baking dish), combine the cooked rice, the remaining coconut milk from the can, the 10.75-ounce can of cream of chicken soup, drained pineapple, diced water chestnuts, cooked chicken, sliced green onions, 3/4 cup sliced almonds, and half of the 8 ounces shredded cheddar cheese (reserve the other half for the topping). Stir until everything is evenly mixed, then spread the mixture evenly in the prepared 9 x 13-inch dish.
Bake the casserole, uncovered, for 30 minutes.
Remove the casserole from the oven. Sprinkle the reserved half of the shredded cheddar cheese over the top, then evenly scatter the 2 cups chow mein noodles over the cheese.
Return the casserole to the oven and bake an additional 8–10 minutes, or until the cheese has melted.
Remove from oven and serve immediately.