Preheat the oven to 425 degrees and put a 9- or 10-inch oven-proof skillet in the oven.
Heat 2 Tablespoons avocado oil in a separate medium skillet over medium heat. Add onion and sauté until caramelized, about 15-17 minutes, lowering the heat as necessary. When the onions are golden, turn the heat back up to medium and add the spinach, season with ¼ salt and pepper, and cook until wilted, about 3-5 minutes.
Meanwhile, add eggs, milk, flour, and ½ teaspoon salt to a blender and blend for 1 minute.
Remove hot skillet from the oven and carefully add the remaining 2 Tablespoons of avocado oil. Pour batter into the skillet with the avocado oil and (using an oven mitt). Arrange onions, spinach, and cheese on top of batter leaving a 1-inch border around the edge of the pan without any vegetables. Bake until puffed and set, about 25-30 minutes.
Serve immediately with desired accompaniments, such as a crisp green salad with a vinaigrette or roasted asparagus or sautéed mushrooms.