Lettuce: Buttercrunch (Lactuca sativa ‘Buttercrunch’). THE ULTIMATE KETO “BREAD”: Famous for its cup-shaped, flexible leaves and crisp rib, Buttercrunch is the #1 choice for low-carb diets. HEAT TOLERANT & SLOW BOLT: While lettuce prefers cool weather, Buttercrunch is bred to stand up to heat better than most varieties.