Slice the chicken sausage into thin half-moon rounds and finely dice the red bell pepper and yellow onion.
Heat the avocado oil in a medium non-stick skillet over medium-high heat until shimmering.
Add the sliced sausage, diced peppers, and onions to the pan and sauté for 5 minutes until the sausage is browned and the vegetables are soft.
While the sausage cooks, whisk the whole eggs, liquid egg whites, sea salt, and black pepper in a bowl until the mixture is uniform and slightly frothy.
Stir the baby spinach into the skillet and cook for 30 seconds until just wilted.
Reduce the skillet heat to medium-low and pour the egg mixture evenly over the sausage and vegetables.
Use a silicone spatula to gently push the eggs from the outer edges toward the center, creating soft folds until the eggs are just set but still moist.
Remove the skillet from the heat, sprinkle with crumbled feta cheese and chopped fresh chives, and serve immediately while hot.