How to Make It
Heat 2 tablespoons melted schmaltz in a large skillet over high until very hot, about 3 minutes. Add livers; sprinkle with salt and pepper. Cook until lightly browned and medium-rare, 1 to 2 minutes per side. Transfer to a plate. Reduce heat to medium. Add onions and 1 tablespoon schmaltz, and stir to coat onions in schmaltz. Cook, stirring occasionally, until onions are caramelized, 15 to 20 minutes. (If onions start to brown too quickly, reduce heat to low.) Stir in rosemary and lemon zest, and cook, stirring constantly, until fragrant, about 30 seconds. Remove from heat, and stir in brandy and sugar, scraping to loosen browned bits from bottom of skillet. Return skillet to heat over medium, and cook 1 minute.
Place onion mixture, livers (do not include any accumulated juices from livers), and egg yolks in a blender; process on high speed until smooth, about 30 seconds, stopping to scrape down sides as needed. With blender running on medium speed, add remaining 1/3 cup schmaltz in a slow, steady stream, processing until completely blended, about 40 seconds. Push liver mixture through a fine wire-mesh strainer into an airtight container; discard solids. (Mousse should be very smooth.) Press plastic wrap directly onto surface. Chill until firm, at least 8 hours or up to 12 hours. Let mousse stand at room temperature 15 minutes before serving. Sprinkle with flaky sea salt, and serve with toasted challah.