Heat a large pan over medium heat and melt butter. Add sliced onions, 1/2 teaspoon salt, and a pinch of pepper.
Cook onions for 7-10 minutes until cartelized and golden brown. Add a few tablespoons of water at a time if pan gets dry to prevent burning.
Add kale and cook with onions for 2-3 minutes until wilted. Remove pan from heat and let cool.
Preheat oven to 350°F and coat a 9-inch pie pan with non-stick spray or butter.
Add eggs, milk, 1/4 teaspoon of salt, and a pinch of pepper to a high-speed blender or large bowl. Mix well until light and fluffy.
Add onions and kale to prepared pan in an even layer. Pour egg mixture onto onions. Bake for 25-30 minutes until top is golden brown and eggs are just set.
Option to add shredded cheese for even more flavor.