The richness of this creamy mousse is balanced nicely by an accompanying tart, crisp pickle; choose one or many.
Melt 2 tablespoons butter in a skillet over medium-high until foamy. Add onion, garlic, and sage; cook until softened, about 2 minutes. Add chicken livers, 3/4 teaspoon salt, and 1/4 teaspoon pepper; cook, stirring occasionally, until browned, 4 to 6 minutes. Add Marsala and bring to a boil; cook 30 seconds, then reduce heat to medium and simmer, covered, until livers are just cooked through, about 3 minutes.
Transfer livers with pan juices, remaining 3 tablespoons butter, and cream to a food processor. Puree until silky, about 2 minutes. Pass mousse through a sieve to remove any lumps. Season with salt and pepper. Transfer to a 1 1/2-cup ramekin or bowl and refrigerate at least 1 hour and up to 2 days.
Remove mousse from refrigerator 15 minutes before serving. Just before serving, spoon jelly over top. Serve, with crostini and pickles.