Tangy buttermilk is the perfect partner for sweet-tart blueberries in this simple make-ahead tart. Martha made this recipe on episode 701 of Martha Bakes.
Sprinkle gelatin over cold water in a small bowl; let stand until softened, about 5 minutes.
Heat cream, sugar, and salt in a small saucepan over medium heat, stirring to dissolve sugar and salt. Add gelatin mixture and stir until it has dissolved. Remove from heat; let cool. Stir in buttermilk and lemon juice.
Remove crust from pan. Spread filling into crust and refrigerate until slightly set, about 15 minutes. Scatter blueberries evenly over top. Refrigerate until filling is firm, about 2 hours (or up to 1 day).