Kale Egg Breakfast Cups

Ingredients (for 6 servings)

• 2 cups — kale, chopped (curly or Tuscan, stems removed)
• ½ cup — bell peppers, diced
• ¼ cup — onion, diced
• 12 whole — large egg whites
• ¼ tsp — salt
• ¼ tsp — pepper
• 1 tbsp — olive oil (for greasing)