Pour the strawberry gelatin powder into a large heatproof bowl. Add 1 cup boiling water. Whisk constantly until the gelatin dissolves and the liquid is smooth.

Stir in 1 cup cold water. Cover and refrigerate until the mixture is slightly thickened but not fully set, about 20 to 30 minutes.

While the gelatin chills, hull and dice the strawberries into bite-size pieces. Place the crushed pineapple in a fine sieve or on paper towels and press to remove excess juice.

When the gelatin is thickened but still pourable, take it from the fridge. Gently fold in the diced strawberries, mini marshmallows, whipped topping, and drained pineapple. If using, fold in the chopped nuts. Mix until everything is evenly distributed.

Pour the combined mixture into a large glass bowl or trifle dish. Smooth the top with a spatula. Refrigerate until fully set, about 2 hours.

Serve chilled. Spoon into bowls and add any optional garnish, like a strawberry slice or a sprinkle of nuts.