Preheat the oven to 375°F. Coat a large rimmed baking sheet with cooking spray.
Heat the olive oil in a skillet over medium heat. Cook the onion and bell pepper until softened.
Add the spinach and cook until wilted. Remove from the heat and stir in the diced turkey sausage.
Whisk the eggs, milk, baking powder, salt, and pepper in a large bowl. Stir in the cooked vegetables and sausage.
Pour the egg mixture onto the prepared baking sheet and spread it evenly. Bake for 22 to 25 minutes, until the center is set.
Cool the baked egg mixture for several minutes, then cut it into twelve squares.
Toast the English muffins. Add one egg square, one cheese slice, and tomato to each muffin. Serve warm or wrap for storage.