Heat oven to 325°F. Set prepared crust in its pie pan on a rimmed baking sheet.
Beat room-temperature cream cheese and mascarpone smooth on medium-low. Mix in sugar, scraping bowl.
Mix in eggs individually on low, then vanilla, optional lemon juice, sour cream or yogurt, and salt. Avoid whipping in excess air.
Pour into crust; bake about 40–55 minutes until edges set and center reaches 160°F, gently wobbly but not liquid. Start checking at 40 minutes.
Cool on rack about 1 hour; refrigerate at least 4 hours before slicing. This egg filling requires baking; never use unbaked.