Mist 24 stick-resistant mini-muffin cups with cooking spray. In a skillet, heat oil over medium-high heat; cook mushrooms and red onion, stirring occasionally, until tender, about 5 minutes. Stir in spinach, salt, pepper and nutmeg; cook, stirring, until spinach is wilted and no liquid remains, 1 or 2 minutes. Divide among prepared muffin cups; sprinkle each with Swiss cheese.
In a blender, blend eggs, milk, flour and baking powder until smooth; pour over mushroom mixture, dividing evenly.
Bake in a preheated 350°F (180°C) oven until puffy and set, 12 to 14 minutes.
Cool for 5 minutes in pan on rack. Serve warm or at room temperature.
Per quiche: 47 calories, 3 g protein, 3 g fat (1 g saturated fat), 3 g carbohydrates, 0 g fibre, 55 mg cholesterol, 48 mg sodium