Position a rack in the center of the oven and heat the oven to 350°F. In a medium bowl, combine the ricotta and cream cheese. Using an electric mixer, beat on medium speed until well blended and no lumps remain, about 3 minutes. Add the sugar, flour, and salt and continue beating until well blended, about 1 min. Add the egg yolks, lime zest, and lime juice. Beat until just incorporated. Use a rubber spatula to scrape the filling into the crust and spread the filling evenly.
Bake the tart until the filling just barely jiggles when the pan is nudged, 30 to 35 minutes. Let cool completely on a rack. Refrigerate the tart in the pan until chilled and firm, 2 to 3 hours. Serve garnished with strips of lime zest, if you like.
Photo: Scott Phillips