Add in onions, garlic, salt, pepper, and crushed red pepper flakes. Cook for 3 minutes. Add the mushrooms and continue cooking for 5 minutes until the mushrooms are soft. Add the kale, cooking it until it is bright green, about 3 minutes.
1 medium yellow onion, 4 garlic cloves, 1 teaspoon kosher salt, 1/4 teaspoon black pepper, 1/4 teaspoon crushed red pepper flakes, 8 ounces cremini mushrooms, 4 cups kale