Wash the kale. Remove the large stems and any discolored leaves. Stack the leaves and slice them crosswise into 1/8-inch strips.
Combine the tomatoes, onions, cumin, garlic, red pepper sauce, and salt in a saucepan, cover, and cook on medium heat for 5 minutes. Add the greens, cover, and gently simmer, stirring frequently, for 10 to 15 minutes, until the greens are tender. Add pepper to taste and serve.