Preheat oven to broil and set rack on lowest level.
Heat oil in a medium skillet over medium-high heat. (Use a skillet that will fit in the oven and has an oven-safe handle.) Add scallions and red bell peppers, and cook for about 2 minutes.
Add garlic, tomatoes, thyme, marjoram, and chicken (or other meat), and sea salt and pepper, if desired. Cook for about a minute.
Add spinach to the skillet and cook for another minute. Do not allow spinach to bunch together.
Add eggs to the skillet. Using a spatula, stir the mixture until the eggs begin to set, but are still runny.
Put skillet in the oven for 4 to 5 minutes, until the frittata becomes firm and fluffy.
Transfer cooked frittata to a plate. Cut into quarters and serve each piece topped with two slices of avocado.