Line a 4-by-6-inch loaf pan with plastic wrap. Melt 2 tablespoons butter in a large skillet over medium-high heat until foamy. Cook shallots and garlic until softened, about 2 minutes. Add chicken livers, 1 teaspoon coarse salt, and the cayenne, and cook, stirring occasionally, for 10 minutes. Remove from heat, and add wine and thyme. Return to heat, and cook, covered, until liquid reduces by half, about 6 minutes.
Pulse chicken livers, pan juices, and remaining 10 tablespoons butter in a food processor until smooth. Season with coarse salt and pepper. Pour into pan, and smooth with an offset spatula; refrigerate until set, about 2 hours.
Preheat oven to 375 degrees. Brush baguette with oil, and toast until golden. Sprinkle with fleur de sel.
Turn out pate from pan. Spread pate and quince jelly on crostini.