Heat 1 Tbsp avocado oil in a large skillet over medium-high heat.
Add chopped chicken thighs and let sit undisturbed for a few minutes to brown, then stir and continue cooking until most liquid has evaporated and chicken is browned, about 5–8 minutes.
Stir the 1/3 cup pesto into the browned chicken until well coated, then add the baby spinach and cook, stirring, until the spinach wilts, about 2–3 minutes.
Taste the chicken mixture and add sea salt as needed.
Heat 2–3 teaspoons avocado oil in a small 6-inch skillet over medium heat, or lightly spray the skillet.
Whisk 2–3 eggs in a bowl with a pinch of sea salt until combined.
Pour beaten eggs into the hot small skillet and cook until the edges firm and the center begins to set, about 2–3 minutes.
Carefully flip the omelette and cook an additional 30 seconds to 1 minute until set to your preference, then transfer to a plate.
Fill the omelette with a portion of the pesto chicken and spinach mixture, then sprinkle with crumbled feta and additional pesto if desired; fold and serve.
