How to Make It
Stir together crème fraîche, mayonnaise, relish, and lemon juice in a medium bowl. Gently stir in 4 teaspoons of the caviar; cover and chill until ready to use.
Gently stir together ground chuck and Worcestershire sauce in a medium bowl; gently stir in remaining caviar. Shape beef mixture into 4 patties; sprinkle both sides with salt and pepper.
Heat oil in a large cast-iron skillet over medium-high. Add the patties; cook until a thermometer inserted in patties registers 145°F (medium-rare), 3 to 4 minutes per side.
Place lettuce on bottom of buns; top with burger patties. Spread caviar mayo onto patties. Top with onion slices and bun tops.