Mash the avocado with the sour cream, lime juice and 1/4 teaspoon salt until mostly smooth, then press plastic wrap against the crema and chill it.
Mix the chicken, scallions, panko, egg, remaining salt and cumin gently until combined.
Shape 4 patties with wet hands and chill them for 10 minutes.
Heat a lightly oiled nonstick skillet over medium and cook the patties for 6 to 7 minutes on the first side.
Flip once and cook for 5 to 7 minutes more until the thickest centers reach 165°F.
Toast the buns in the cleared skillet for 1 minute.
Spread avocado crema over both halves, then build each burger with lettuce, tomato and one chicken patty.
