Place the kale in a large bowl and dress very lightly with olive oil so the leaves are barely coated. Season with salt. Spread the leaves out across 2 roasting sheet trays so they aren't stacked up on each other too much. Roast in the oven until crispy but still green, 12 to 15 minutes. Remove from the oven and dress with a squeeze of fresh lemon juice and a touch of crushed red pepper flakes.