Slice the mushrooms and dice the red pepper. In a bowl, whisk the egg whites, the whole egg and the pepper until frothy, about 20 seconds.
Heat a 20 to 25 cm (8 to 10 inch) non-stick skillet over medium heat and coat with cooking spray.
Add the mushrooms and pepper and cook, stirring, until soft and the mushroom liquid has evaporated, about 3 minutes. Add the spinach and stir until wilted, about 30 seconds. Tip the vegetables onto a plate.
Wipe the pan, spray again and return it to medium-low heat.
Pour in the egg mixture. Let it cook undisturbed until the edges begin to set, about 1 minute, then lift the edges with a spatula and tilt the pan so the raw egg runs underneath. Repeat until the top is almost set, about 3 minutes in total.
Spoon the vegetables and crumbled feta over one half. Fold the other half over the filling and cook 30 seconds more so the centre is fully set (74°C / 165°F).
Slide onto a plate and serve with the toast.
Tips
One whole egg gives the omelette colour and stops it tasting flat.