In a large mixing bowl, combine the sweetened condensed milk and sour cream. Whisk until smooth and creamy, about 2 minutes. Gradually add the lemon juice while whisking until the mixture thickens slightly.

Pour the thick filling mixture into the prepared graham cracker crust and smooth the top with a spatula.

Preheat the oven to 350°F (175°C). Bake the pie for 6 to 8 minutes to help set the filling, then allow it to cool completely.

Transfer the pie to the refrigerator and chill for at least 4 hours until the filling is cool and firm to the touch.

In a mixing bowl, beat the heavy whipping cream and powdered sugar on medium-high speed until stiff peaks form, about 3-5 minutes.

Spread the whipped cream on top of the chilled pie. Optionally, garnish with fresh lemon slices or zest before serving.