• 1 cup fresh lemon juice
• 3/4 cup (1 1/2 sticks) unsalted butter, cut into 1/2-inch cubes
• Nonstick vegetable oil spray
• 2 cups shortbread cookie crumbs (about 7 1/2 ounces)
• 1/4 cup (1/2 stick) unsalted butter, melted
• 4 teaspoons unflavored gelatin
• 1 1/2 cups chilled heavy whipping cream
• Lemon slices, cut into quarters (garnish)