Into a large pan over medium heat add butter and diced onions and cook for 2-3 minutes, or until onions are soft. Add kale, spinach, parsley, garlic, and the blistered tomatillos to the pan and cook for an additional 6-8 minutes, or until all of the greens are wilted. Stir in salt, cumin, red chili flakes, black pepper, and lemon juice until incorporated.