Add 3 oz strawberry Jell-O to a medium bowl. Stir in 1 cup boiling water until fully dissolved, then mix in 1 cup cold water. Refrigerate for about 90 minutes, until the Jell-O is slightly thickened but not fully set.

Once the Jell-O is almost set, gently fold in 1 cup sliced strawberries and 3 cups thawed whipped topping until smooth and evenly combined.

Tear the 8-inch angel food cake into soft chunks and fold the pieces into the strawberry mixture, making sure the cake is evenly coated so it can absorb the flavor.

Pour the mixture into a 9×12-inch casserole dish and lightly press down any cake pieces to help them soak up the liquid. Top with additional sliced strawberries, cover, and refrigerate for at least 1-2 hours before serving cold.