In an oven-safe skillet, heat oil over medium heat; add bacon. Cook, stirring frequently, until crisp, about 5-7 minutes; drain bacon on paper towels and set aside.
Reduce heat to medium-low; add shallots. Cook, stirring frequently, until softened, about 2-3 minutes; add a few handfuls of kale, tossing with tongs and adding more as it cooks down, until kale is tender, 3-5 minutes. (If skillet seems dry while kale is cooking, add a splash of water.)
In a large bowl, whisk together eggs, egg whites, bacon, cheese and salt; pour over kale mixture. Stir to evenly distribute ingredients; cook undisturbed until set, about 3-5 minutes.
Transfer skillet to oven; broil until browned, 30 seconds to 1 minute.
Serving size: ¼ of frittata
Garnish with chopped dill and scallion, if desired.