Make the grapefruit curd. Combine the grapefruit zest and juice, sugar, and eggs in a heavy-bottomed saucepot. Whisk the mixture together until well combined. Heat the curd over medium-low heat. When the mixture starts to gently simmer, start adding the butter bit by bit, whisking constantly. Continue to simmer the curd until it begins to thicken, stirring frequently, for about 10 minutes. Remove from heat and transfer the curd to a clean bowl. Place plastic wrap directly over the curd, touching the surface to prevent a skin from forming. Let cool completely. You can prepare the curd up to 3 days in advance.

While the grapefruit curd is cooling, make the crust. Combine the graham cracker crumbs with poppy seeds, brown sugar, lemon zest, and melted butter. Press the mixture into the prepared baking dish to form an even crust. Par bake the crust for 8 minutes. Remove from the oven and let cool.

Decrease the oven temperature to 325 degrees F.

Make the cheesecake filling. Beat the cream cheese and sugar together in the bowl of a stand mixer until light and fluffy. Add the eggs one at a time, fully incorporating the egg before adding another. Add the sour cream and vanilla and beat until combined.