Beat the eggs together with milk, Chia seeds and black pepper. Set the mixture aside for about 10 minutes. Heat a small non stick omelette pan with olive oil and pour the mixture into the pan. Tilt the pan around to ensure egg mixture completely covers the pan and cook under a medium heat until the omelette is almost cooked. Crumble the feta over the surface with the spinach and continue to cook until the spinach has slightly wilted. Fold the omelette in half and serve.