Preheat the oven to 190°C (375°F) with a rack in the centre. Spray a 23 x 33 cm (9 x 13 inch) rimmed baking sheet or dish well with cooking spray.
Heat a skillet over medium-high heat and cook the turkey sausage, breaking it up, until browned and it reaches 74°C (165°F), about 6 minutes. Drain on paper towel.
Scatter the sausage, diced peppers and spinach evenly over the tray.
Whisk the eggs, egg whites, milk and pepper until smooth, then pour over the filling and sprinkle with the cheddar.
Bake for 20 to 22 minutes, until puffed and set in the centre with no liquid egg when you press it, 74°C (165°F) inside.
Cool for 5 minutes, then cut into 6 squares. Keeps 4 days in the fridge; reheat for 60 to 90 seconds in the microwave.