This egg salad sandwich is loosely based on an open-faced egg salad sandwich served at a local bistro. The unexpected pairing of pesto and Jarlsberg compliments the egg salad tremendously. The amounts of each ingredient can vary depending on taste
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels; crumble.
Preheat oven broiler and set rack about 6 inches from heat source.
Mix eggs, mayonnaise, mustard, salt, and pepper in a bowl; fold in crumbled bacon.
Toast sourdough bread under broiler until lightly browned, about 2 minutes. Spread pesto evenly on each slice of bread; top with 1/4 egg salad mixture and spread to cover completely. Top each sandwich with 1 slice Jarlsberg cheese.
Return sandwiches to broiler and cook until cheese is melted, 2 to 3 minutes. Served sandwiches warm and open-faced.